feat(store): Phase 2A — Postgres store with migrations and CRUD

[smallchungus]

Summary

Phase 2A of the queue: durable state layer.

• Initial migration creates pipeline_jobs and job_status_history tables (Phase 3 will ALTER TABLE for Gmail-specific columns).
• Store type wrapping *pgxpool.Pool with the queue's CRUD primitives:
  • EnqueueJob, GetJob, ClaimJob (atomic queued→running), MarkDone, MarkFailed (with retry/dead branching at attempts >= max_attempts).
• Migrate(ctx, dsn) applies embedded SQL migrations via golang-migrate + iofs source.
• 9 integration tests against real Postgres via testcontainers-go (no mocks).

Test plan

• make lint (0 issues)
• make test-unit
• make test-integration (9 store tests + 2 testutil smoke tests)
• make k8s-validate (8 resources, all valid)
• CI green