test(backend): fail the build when the compose file lacks a required variable (#107)

The one-line compose fix in the previous commit unblocks QA. This is the part that stops it happening again, and it is the more useful half.

The failure was not really a missing variable. It was that nothing connected two files: envValidation.ts gained a required variable, docker-compose.qa.yml did not set it, and nothing noticed until a container refused to boot on deploy. CI passed the whole time, because CI supplies its own environment and never reads the compose file — which is exactly why "CI is green" was the wrong evidence to have offered.

So a unit test now reads the compose file and asserts it sets everything the validator demands. It imports ALWAYS_REQUIRED rather than restating it, which is the only version of this test worth having: a copied list would pass forever while the next variable added to the validator went unguarded in precisely the same way.

Two further assertions earn their place. UPLOADS_DIR is hardcoded rather than taken from a stack variable on the grounds that it must agree with the volume mapping, so the test checks it against the mount rather than leaving that a claim in a comment. And ADMIN_GATE_SECRET must be present as an interpolation rather than a literal, since a secret in the repository would defeat the point of having one.

There is also a test guarding the test: a regex that matched nothing would make every other assertion in the file vacuously true, so one case asserts that parsing found entries at all.

Fired deliberately rather than assumed. Removing the UPLOADS_DIR line reproduces the original failure as two failing tests; restoring it returns to ten passing. A guard that has only ever been observed passing is not known to guard anything.

What this cannot do is check production, which runs from a Portainer stack outside this repository. That gap is now written into the README beside the validation rules, along with the reason a variable set only in Portainer's stack UI never reaches the container: stack variables are interpolated into the compose file, not handed to the service.

172 unit tests pass, lint unchanged at 4 warnings.

Refs #107
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -97,6 +97,10 @@ mkdir -p /tmp/redefined-uploads
 
 `PUBLIC_URL` is required only alongside SMTP because its only job is building links in email; an environment that cannot send mail does not need it. `UPLOADS_DIR` has no such reprieve — its fallback of `/app/uploads` is correct inside the container and wrong everywhere else.
 
+**Adding to the required list has reach beyond this repository.** A variable added to `ALWAYS_REQUIRED` must also be set in every environment that deploys, and there are two of those. `docker-compose.qa.yml` is checked automatically — `backend/tests/unit/composeEnvironment.test.ts` reads the validator's own list and fails if the compose file does not set something on it, which is what #107 existed to prevent from recurring. Production runs from a Portainer stack **outside this repository**, so nothing can check it: that one has to be updated by hand, before the deploy rather than during it.
+
+Note also that setting a variable in Portainer's stack environment is not the same as giving it to the container. Stack variables are interpolated into the compose file as `${VAR}`; a service receives exactly what its own `environment:` block lists. A variable with no line there never arrives, however carefully it was set in the UI.
+
 **Note (PowerShell):** environment variables set with `$env:` only last for the current terminal session/tab. If you close and reopen VS Code's terminal, you'll need to re-run step 3 before starting the backend again.
 
 ### 4. Run the backend