Adds a shared-token auth dependency for the viewing API (Task 2.9). Trust
model: when API_TOKEN is empty (the default) auth is disabled — we trust the
network boundary (local/intranet tool). When set, every request must present
the token via `Authorization: Bearer <token>` or `?token=<token>`, else 401.
`/api/health` is exempt so liveness probes work without the token.
- app/config.py: add `api_token: str = ""` to Settings.
- app/api/auth.py: `require_auth` dependency (secrets.compare_digest, reads
get_settings() per-request so token rotation via cache_clear() works).
- app/api/main.py: apply the dependency globally; document the trust model.
- .env.example: document API_TOKEN.
- tests/test_api_auth.py: 8 tests covering disabled/correct/missing/wrong/
malformed-header/query-param/health-exempt paths.
Co-Authored-By: Claude <noreply@anthropic.com>
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