Add testcontainers-python fixtures for PostgreSQL tests

[peteski22]

Part of #257 — Phase 2 step (4).
RFC: #275.

Blocked by the PostgresStore implementation Phase 2 child.

Scope

Add to server/backend/pyproject.toml dev dependencies:

• testcontainers[postgres]>=4.14.2,<5 (current latest at the time of
  writing — bump to latest at PR time)

Regenerate and commit uv.lock.

Provide a session-scoped pytest fixture that starts a PostgreSQL
container, runs migrations, and yields a PostgresStore connected to
it.

Definition of Done

• Fixture available and used by at least one Postgres-only test.
• Full test suite green locally with Docker available.
• Lint clean.
• README note for contributors: tests requiring Postgres need Docker
  running; SQLite-only tests still work without it.