Wire `CQ_DATABASE_URL` and store factory

[peteski22]

Part of #257 — Phase 1 step (6).
RFC: #275.

Goal

Add the CQ_DATABASE_URL environment variable as the canonical way to
select the backend, with a factory that returns the right concrete
store. Backward-compatible: falls back to CQ_DB_PATH.

Scope

• create_store(database_url: str) -> Store factory in
  store/__init__.py. URL parsing dispatches on scheme:

  • sqlite:///... → SqliteStore
  • postgresql+psycopg://... → raises NotImplementedError for now,
    with a message pointing at the Phase 2 PostgresStore child.

• app.py lifespan resolves the URL:

  database_url = os.environ.get(
      "CQ_DATABASE_URL",
      f"sqlite:///{os.environ.get('CQ_DB_PATH', '/data/cq.db')}",
  )

• Document the new env var in the relevant config doc.

Acceptance criteria

• Existing deployments using CQ_DB_PATH continue to work unchanged.
• Setting CQ_DATABASE_URL=sqlite:///custom.db works.
• Setting a postgresql+psycopg://... URL fails with a clear
  NotImplementedError message pointing at the Phase 2
  PostgresStore child.

Definition of Done

• TDD: failing tests for each branch (default fallback, explicit
  SQLite URL, Postgres URL → NotImplementedError) land before the
  factory implementation.
• End-to-end smoke: server boots under each of (a) no env, (b)
  CQ_DB_PATH only, (c) CQ_DATABASE_URL=sqlite:///....
• Full test suite green.
• Lint clean.
• Deployment / configuration docs updated to introduce
  CQ_DATABASE_URL and document the CQ_DB_PATH fallback as legacy.
  (Full multi-backend deployment guide lands in the Phase 4
  documentation child.)

Blocked by

• Implement SqliteStore against Store protocol via Core helpers
  (Phase 1 child).