test(backend): add the assertions that mutation testing shows are missing #409

Follow-up to #169 / #399. The first full mutation run left 4351 survivors across 583 functions, now in backend/tests/mutation-baseline.txt. Survivors cluster in a few areas where tests pass without pinning the behaviour. Work through them by group: add the missing assertion, rerun the group with its glob, and lower its baseline lines.

Groups, by risk

1. Auth flows. OAuth login and associate callbacks (60 and 52 survivors), complete_mfa_challenge (41); app.api.auth.services survives 29% overall.
   just mutation 'app.api.auth.services.oauth*' 'app.api.auth.services.mfa*'
2. Upload safety. ClamAVScanner.scan (34) and recompute_user_upload_quota (32).
   just mutation 'app.api.file_storage.upload_*'
3. RPi camera plugin, 38% survival, the highest of any large package: cross_worker_relay._execute_and_respond (91), YouTube livestream and recording setup (73, 62), capture_and_store_image (46).
   just mutation 'app.api.plugins.rpi_cam*'
4. Error details and keys. About 1150 survivors change a string the code relies on: detail= and reason= arguments, dict keys, return values, exception messages. Tests check the status but not the message (see "Assertions" in backend/tests/README.md).

Router factories (build_categorized_admin_router, the OAuth build_*_router) survive mostly through OpenAPI summary and description strings; leave those in the baseline.

Also

• mutmut skips decorated functions, so route handler bodies are never mutated. The handlers with real logic are a handful of rpi_cam ones (receive_camera_upload, receive_preview_thumbnail_upload, claim_pairing_code, poll_pairing_status, proxy_hls). Either check their guards with the if False: method or move the logic into plain functions.
• Rebaseline per function by editing its line; just mutation-baseline replaces the whole file and needs a full run.