feat(backend): add an application-layer gate to the admin API (#63)

Authorization for the admin panel and the admin API has lived entirely in one auth_request regex in an Nginx Proxy Manager config outside this repository. That control is real and it works — nothing is publicly exposed today — but it is invisible from the code, untested here, and not reviewed when this code changes. Three things follow from that, and the first is the one worth the change.

An admin route added at a path the regex does not match is unprotected the moment it is written, and nothing in Express indicates that. Anything reaching the published container port directly bypasses authentik entirely. And locally there is no gate at all, so no developer ever sees the boundary being enforced.

requireAdminGate is attached to each admin router rather than to a path prefix, which is what makes it useful rather than merely redundant with the proxy. An admin router added later at some other path inherits the gate; because the proxy only injects the header on paths its regex matches, that router refuses on its first request instead of being quietly public. A 403 in that situation is the boundary reporting that it has drifted.

The gate is optional, and unset means exactly today's behaviour. That keeps local development and all 113 existing admin test call sites working untouched, and means shipping the image before configuring the proxy cannot take the admin panel down. What it does not do is stay silent about it: the server warns at boot when the gate is inactive, naming what is unprotected. This project has been bitten repeatedly by controls that report success while doing nothing, and an unconfigured gate should be a visible choice rather than an invisible one.

An empty value is treated as unset rather than as a secret, because enforcing an empty secret would admit any caller sending an empty header. Comparison is timing-safe over SHA-256 digests of both sides: timingSafeEqual throws on buffers of unequal length, so comparing raw values would turn a short header into a 500 rather than a 403, and a length check first would leak the secret's length.

Turning it on requires the secret in two places at once — the stack environment and a proxy_set_header line on the gated location in NPM. Setting only one gives 403s until the other catches up. That coupling, and the three consequences above, are now written into the README beside the deployment section, since none of it is visible from the code.

Verified over real HTTP as well as in tests. Booting without the secret logs the warning and serves admin normally; booting with it returns 403 for a missing header, 403 for a wrong one, 200 for the right one, and leaves the public storefront at 200 throughout, with each refusal logged distinguishably and without echoing the value it was sent. 8 new unit tests, 9 new integration tests covering every admin router separately — a correct middleware nobody mounted would pass the unit tests and protect nothing. 106 unit and 153 integration passing, lint 0 errors and 8 warnings unchanged.

Refs #63
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -179,6 +179,27 @@ Production runs as a single Docker image (multi-stage build — the frontend is
 
 The container applies pending migrations before starting the server, so deployed code can never be ahead of the database schema. A failed migration stops the container rather than letting it serve against a schema it doesn't match — check `docker logs` on the app container if it doesn't come up.
 
+### The admin authorization boundary
+
+Worth reading before adding any admin route, because the control is invisible from the code.
+
+Authorization for the admin panel and the admin API lives in a single `auth_request` regex in the Nginx Proxy Manager config — `^/(admin|api/admin)` in production, and `location /` in QA, where the whole site is gated. That config is not in this repository. Three consequences follow, and none of them are visible from Express:
+
+- **An admin route added at a path the regex does not match is not covered by it.** `/api/reports` or `/api/internal/...` would be publicly reachable the moment it shipped.
+- **Anything that reaches the container directly bypasses authentik entirely**, because the gate is in the proxy in front of it. QA publishes port 32751 on the NAS and production publishes its own.
+- **Locally there is no gate at all**, so `/admin` and the whole admin API are open by design and no developer ever sees the boundary being enforced.
+
+`ADMIN_GATE_SECRET` is the application-layer half of this, and it is optional:
+
+| State | Behaviour |
+| --- | --- |
+| Unset | Every admin route is reachable, exactly as before. The server logs an `[admin-gate]` warning at boot saying so, so the state is visible rather than silent. This is what local development and the test suite run in. |
+| Set | Every admin router requires an `X-Admin-Gate` header matching the value, and returns 403 without it. |
+
+To turn it on, the secret has to be set in **two places at once** — the stack's environment, and a `proxy_set_header X-Admin-Gate "<secret>";` line on the gated location in Nginx Proxy Manager. Setting it in only one of them makes the admin panel return 403 until the other catches up. That failure is loud and recoverable, unlike the one it replaces.
+
+The middleware is attached to each admin **router** rather than to a path prefix. That is deliberate: an admin router added later at some other path inherits the gate, and because the proxy only injects the header on paths its regex matches, that router refuses on its first request rather than being quietly public. A 403 in that situation means the proxy regex needs widening — it is the boundary telling you it has drifted.
+
 ### Promoting a reviewed change to production
 
 Only after the change has been reviewed in QA.