chore(docker): expose Postgres on host port 5434 for local test access

Mirrors the agentception pattern: bind the Postgres container's 5432 to 127.0.0.1:5434 on the host so the local test suite and Alembic CLI can reach the DB without Docker networking tricks.

Port choice: 5432 — reserved for any locally-installed Postgres 5433 — already used by agentception's Postgres container 5434 — free; musehub-specific

Changes: docker-compose.yml — add ports: ["127.0.0.1:5434:5432"] to postgres service Makefile — LOCAL_PG_URL points to localhost:5434; all test targets export DATABASE_URL automatically so 'make test' runs Alembic smoke tests without extra setup .env.example — document the host-side URL and the port rationale

Verified: all 4 Alembic smoke tests pass against local Docker Postgres.