Missing `requirements.txt` entry → symptomless rollback

Symptom

Only the newly added route 404s. The app itself is fine (every other route 200). git push succeeded, the file and the registration line exist on the remote, local import passes on the server. Wait eight minutes: still 404.

Cause

requests was missing from requirements.txt. On a clean build the new module fails to import → app fails to start → the PaaS keeps serving the last successful build. Hence "app is healthy, only the new route is missing."

Why it's easy to miss

Existing code was already using requests without it being in requirements. It only worked because it was cached in the existing container. It only blows up on a clean build.

Iron rules

1. When adding a module, diff the import list against requirements.txt
2. New route 404 after deploy → suspect dependencies first (not a lost commit)
3. A successful push is not evidence of a successful deploy. Hit the real URL.
4. A successful local import is not evidence of a successful deploy.

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