Feat/photo worker tests

[ademboukabes]

This PR adds comprehensive unit tests for the async PhotoWorker (app/worker/photo_worker/main.py) to secure the background processing pipeline.

Key Additions:

• Fully mocked infrastructure: Safely isolates the tests from MinIO, NATS, Postgres, and the AI Model.
• Success Workflows: Covers logic for 0 faces, single faces, and group faces (including Firebase push notifications generation).
• Error Boundaries: Verifies graceful failure handling when MinIO network errors or AI crashes (e.g. Out Of Memory) occur.
• Retry Logic: Tests the exponential backoff retry mechanism for MinIO downloads.

This boosts the test coverage for the core AI background tasks and prevents future regressions.