test(api): isolate container DB between tests

[escape0707]

Summary

Part of #32454 (comment). Draft follow-up to #36308.

This adds a central DB isolation fixture for container integration tests. After each test that used the containerized Flask app, the fixture truncates SQLAlchemy application tables and restarts identities.

The cleanup transaction sets SET LOCAL lock_timeout = '5s' so leaked sessions or open transactions fail CI loudly instead of causing a long hang.

Design

• Function-scoped cleanup: each test may commit freely, but the next test should start from a clean application DB.
• Autouse fixture does not directly depend on flask_app_with_containers, so tests that do not use the containerized app do not start the full stack just because the cleanup fixture exists.
• Scope is limited to SQLAlchemy tables in db.metadata; Redis, object storage, and custom ad hoc metadata remain separate cleanup concerns.

Review order

Please review and merge #36308 first. This branch is stacked on top of that service fix branch; after #36308 merges, this PR should reduce to the container DB isolation change.

A diagnostic version of this cleanup is recorded in #36304. That draft intentionally applies the cleanup before the session fixes, so it can expose the existing leaked-session failure mode.

Validation

• uv run --project api ruff check api/services/attachment_service.py api/services/file_service.py api/tests/test_containers_integration_tests/conftest.py
• uv run --project api python -m compileall -q api/services/attachment_service.py api/services/file_service.py api/tests/test_containers_integration_tests/conftest.py

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	43.69%	+43.69%
Strict coverage	0.00%	43.22%	+43.22%
Typed symbols	0	22,136	+22,136
Untyped symbols	0	28,840	+28,840
Modules	0	2558	+2,558