Add psycopg v3 dependency and `postgresql+psycopg` URL handling

[peteski22]

Part of #257 — Phase 2 step (1).
RFC: #275.

Blocked by Phase 1 completion (in particular #309 — Wire
CQ_DATABASE_URL and store factory).

Scope

Add to server/backend/pyproject.toml:

• psycopg[binary,pool]>=3.3.3,<4 (current latest at the time of
  writing — bump to latest at PR time)

Regenerate and commit uv.lock. The lockfile (with hashes) is the
supply-chain control; pyproject.toml bounds express compatibility
intent.

Extend the create_store() factory (introduced in #309) to dispatch
postgresql+psycopg://... URLs at a PostgresStore stub raising
NotImplementedError with a pointer at the Phase 2 implementation
issue.

Definition of Done

• TDD: failing test for the Postgres branch lands first.
• Full test suite green.
• Lint clean.
• uv.lock committed.