Implement `PostgresStore` against `Store` protocol

[peteski22]

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

Blocked by the psycopg dependency / URL handling Phase 2 child.

Scope

Implement PostgresStore in
server/backend/src/cq_server/store/_postgres.py against the Store
protocol (defined in #306). Use the shared SQLAlchemy Core helpers
(#307) for portable queries; own connection-pool lifecycle and any
Postgres-native specifics.

Migration locking via pg_advisory_lock is handled separately in the
next Phase 2 child.

Definition of Done

• TDD: failing tests covering each protocol method first, then the
  implementation. Tests run against the testcontainers-postgres
  fixtures (added in the testcontainers Phase 2 child).
• Full test suite green on both backends.
• Lint clean.
• No public docs change required at this step (deployment docs land
  in Phase 4).