docs: design for adding ESLint to both workspaces (#60)

Records the measurement the design rests on — 435 violations from a full-strength config, of which 325 are the no-unsafe-* family — and the decision not to enable recommendedTypeChecked, since those 325 all trace to untyped pool.query rows and fetch responses, which is the whole of #65. Also records two of the issue's premises that measurement contradicts: exhaustive-deps flags 2 rather than the 10 the issue inferred from empty dependency arrays, and the backend is already clean on the defect rules because #59 wrapped every async route. Refs #60
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 # ESLint — Design
 **Issue:** [#60 — No ESLint anywhere](https://gitea.bermudalamb.synology.me/bermudalamb/redefined-designs/issues/60)
 **Date:** 2026-08-19
 **Status:** Approved
 ## Goal
 Put a linter in front of the code that catches, at authoring time, the classes of defect this
 project has already shipped and then fixed by hand. Two workspaces, one CI job, green on the day it
 lands.
 The issue's own framing is the target: TypeScript's `strict: true` checks types and nothing else, so
 `rules-of-hooks`, `exhaustive-deps`, `no-floating-promises` and the SonarJS rules currently run
 either never or on a server after the fact.
 ## What the code actually reports
 Every number below comes from running the candidate rule set against `backend/src` and
 `frontend/src` before writing any config, rather than from estimating. A full-strength config —
 `typescript-eslint` recommendedTypeChecked, `sonarjs`, `react-hooks` v7, `jsx-a11y` — reports **435
 violations across 50 files**.
 | Rule family | Count | Disposition |
 | --- | --- | --- |
 | `@typescript-eslint/no-unsafe-*` (5 rules) | 325 | **Deferred to #65** |
 | `no-floating-promises` | 30 | Error — fix here |
 | `no-misused-promises` | 28 | Error, but 25 disappear under `checksVoidReturn: { attributes: false }` |
 | `sonarjs/prefer-read-only-props` | 16 | Warn |
 | `react-hooks` compiler rules (`set-state-in-effect`, `purity`) | 9 | Warn |
 | `no-explicit-any`, `no-unnecessary-type-assertion` | 11 | Warn |
 | `sonarjs/no-nested-conditional`, `cognitive-complexity` | 7 | Warn |
 | `react-hooks/exhaustive-deps` | 2 | Error — fix here |
 | `jsx-a11y/alt-text` | 2 | Error — fix here |
 | `rules-of-hooks` | 0 | Error (nothing to fix) |
 | Remaining singles | 5 | Warn |
 ### Two of the issue's premises do not survive measurement
 **`exhaustive-deps` flags 2, not 10.** The issue inferred ten suspects from ten empty dependency
 arrays. The rule's own analysis clears eight of them — an empty array is only wrong when the effect
 closes over something that changes, and most of these genuinely do not. The two real ones are
 `Admin.tsx:62` (missing `load`) and `Cart.tsx:132` (missing `refreshCartContext`). The "decide case
 by case rather than blanket-autofix" instinct in the issue is still right; it applies to two cases.
 **The backend is already clean on the defect rules.** #59 wrapped every async route, and it shows:
 zero floating promises in `backend/src`. Its three `no-misused-promises` hits are all benign, and
 each was read rather than assumed — `asyncRoute.ts:13` is the wrapper performing its entire purpose,
 and the two `server.ts` scheduled jobs (`setInterval` cart sweep, `cron.schedule` reminder emails)
 already `try`/`catch` internally, so no rejection escapes.
 ## Decisions
 | Area | Decision |
 | --- | --- |
 | Config format | Flat config (`eslint.config.js`), one per workspace |
 | Config location | `backend/` and `frontend/` separately — not a root config |
 | Type-aware linting | On, but narrowly: `recommended`, **not** `recommendedTypeChecked` |
 | `no-unsafe-*` family | Deferred to #65 |
 | `no-misused-promises` | `checksVoidReturn: { attributes: false }` |
 | React Compiler rules | Warn |
 | CI | New `lint` job in `tests.yml` |
 | Fail/warn split | Defect rules error; stylistic and advisory rules warn |
 ### Why not `recommendedTypeChecked`
 It is the obvious choice and it is the wrong one here. Its `no-unsafe-*` family accounts for 325 of
 the 435 findings — 75% — and every one of them traces back to two untyped boundaries: `pool.query()`
 returning `any` rows on the backend, and `fetch(...).json()` on the frontend. Fixing them means
 typing those boundaries, which is the entire content of
 [#65](https://gitea.bermudalamb.synology.me/bermudalamb/redefined-designs/issues/65).
 Turning them on here would produce a lint run whose output is 75% another issue's backlog, which is
 the reliable way to teach everyone to ignore lint output. Instead this issue enables `recommended`
 plus the two type-aware rules that catch defects rather than describe type debt:
 ```js
 '@typescript-eslint/no-floating-promises': 'error',
 '@typescript-eslint/no-misused-promises': ['error', { checksVoidReturn: { attributes: false } }],
 ```
 Both need type information, so `parserOptions.projectService` is still required — the cost of
 type-aware linting is paid either way. This is a choice about which findings to surface, not about
 whether to run the type-aware engine.
 #65 turns the `no-unsafe-*` rules on as it types those boundaries. That is the change that makes
 them green, so that is the change that should own them.
 ### Why `checksVoidReturn: { attributes: false }`
 Left at its default, `no-misused-promises` flags every `onClick={async () => …}` — 25 of its 28
 hits, all antd buttons in the admin screens. Passing an async function to a React event handler is
 idiomatic and safe when the function handles its own errors, which these do. typescript-eslint's own
 documentation recommends this option for React codebases. Without it the rule is 89% noise and would
 be switched off within a week; with it, the three remaining hits are all real signal.
 ### Why the React Compiler rules only warn
 `eslint-plugin-react-hooks` v7 ships the React Compiler rule set — `set-state-in-effect`, `purity`,
 `static-components` and others — alongside the two classic rules. This is React 18 with no compiler
 in the build, so those rules are advising on a stricter model than the code is written against.
 They are not worthless: `purity` correctly flags `Date.now()` being called during render in
 `Cart.tsx:160`. That is a genuine render-purity smell worth seeing. It is not worth failing a build
 over in a codebase that never opted into the model, and fixing all nine would mean restructuring
 effects that behave correctly today — behaviour change well outside what #60 asked for.
 Warn keeps the finding visible and the build honest.
 ### Why per-workspace configs rather than one at the root
 CI already runs `npm install` separately in `backend` and `frontend`, and the two have genuinely
 different rule needs — React and a11y rules are meaningless in `backend/src`, and each workspace has
 its own `tsconfig.json` that type-aware linting must point at. A root config would need file-pattern
 overrides to express a split the directory structure already expresses, and would put React plugins
 in the backend's dependency tree for no reason.
 The cost is a small amount of duplication between the two config files. That is cheaper than the
 alternative, and the files are short.
 ## Fail/warn split
 The issue asks whether to fail CI on day one or start as warnings, and names the middle path: fail
 on the rules with real defect-catching value, warn on the rest. That is what this does.
 **Error — build fails (≈37 sites to fix):**
 | Rule | Sites | Fix |
 | --- | --- | --- |
 | `no-floating-promises` | 30 | `void load()` |
 | `no-misused-promises` | 3 | Targeted disable with a reason |
 | `exhaustive-deps` | 2 | Triage individually |
 | `jsx-a11y/alt-text` | 2 | Add `alt` |
 | `rules-of-hooks` | 0 | — |
 **Warn — visible, does not block (≈73 sites):** SonarJS complexity and style, remaining jsx-a11y,
 React Compiler rules, `no-explicit-any`.
 No `--max-warnings` flag is needed: ESLint exits non-zero on errors and zero on warnings, so the
 split falls out of rule severity alone.
 ### On `void load()` being a real fix rather than a disguised suppression
 Thirty floating promises sounds like thirty bugs; it is not, and the distinction matters because
 `void` can be either an honest annotation or a way to silence a rule without thinking.
 These are fire-and-forget calls to `load()`-style `useCallback`s in effects and event handlers. The
 callbacks were read before choosing this fix: each one sets its own error state — `App.tsx`'s `load`
 calls `setFailed(true)` — so a rejection is already handled inside, and nothing is being swallowed.
 `void` states "deliberately not awaited," which is exactly true here.
 Had any of them lacked internal handling, the fix for that one would be a `.catch`, not a `void`.
 The implementation plan checks each site rather than applying `void` mechanically.
 ## Files
 | File | Change |
 | --- | --- |
 | `backend/eslint.config.js` | New |
 | `frontend/eslint.config.js` | New |
 | `backend/package.json` | `lint` script, devDependencies |
 | `frontend/package.json` | `lint` script, devDependencies |
 | `.gitea/workflows/tests.yml` | New `lint` job |
 | `frontend/src/**` (10 files) | ≈37 violation fixes |
 | `backend/src/asyncRoute.ts`, `server.ts` | 3 targeted disables |
 | `.claude/project-context.md` | Record the linter and the deferral to #65 |
 ## Testing
 The linter is its own test: `npm run lint` in each workspace must exit 0. Success criteria:
 1. `npm run lint` passes in both workspaces with zero errors.
 2. The warning count is reported, not zero, and that is expected.
 3. `npm run build` still passes in both workspaces.
 4. Backend unit and integration suites still pass — the `void` and disable edits touch frontend
   files almost exclusively, but `server.ts` and `asyncRoute.ts` are on the backend.
 5. Playwright e2e passes, since ≈37 edits land in rendered components.
 6. The `lint` job runs in CI and fails the build when a rule in the error set is violated.
 Point 6 is verified by deliberately introducing a violation locally and confirming a non-zero exit,
 not by assuming the job's configuration is correct.
 ## Out of scope
 - **The `no-unsafe-*` family and the `any` backlog** — #65.
 - **Prettier or any formatting tool.** The issue asks for defect detection. Formatting is a separate
  argument with separate churn.
 - **Linting `tests/` and Playwright specs.** Worth doing; a second step once the source is green, so
  that this change's error count stays the measured one.
 - **Replacing SonarQube.** The CI scan stays. This puts the same rule families in the editor, where
  they are cheaper to act on.
 ## Follow-ups this creates
 - **#65 owns turning on `no-unsafe-*`** as it types `pool.query` results and `fetch` responses.
 - **`routesAreWrapped.test.ts` can retire** once `no-misused-promises` runs on the backend in CI.
  The test predates the linter and covers the same ground by scanning source; it is more targeted
  and its failure message is better, so this is a judgement call for whoever next touches either.
  Recorded here so the duplication is deliberate rather than forgotten.