Document `CQ_DATABASE_URL` in deployment / configuration docs

[peteski22]

Part of #257 — Phase 4 step (2).
RFC: #275.

Blocked by:

• The CQ_DATABASE_URL factory Phase 1 child.
• The PostgresStore implementation Phase 2 child.

Scope

Document CQ_DATABASE_URL in the deployment / configuration docs
with concrete examples for:

• SQLite (default; legacy CQ_DB_PATH shown as still-supported
  fallback).
• PostgreSQL via postgresql+psycopg://user:pass@host/db.

Supersedes the brief operator notes added incrementally in earlier
children (consolidate into a single canonical reference).

Definition of Done

• Docs build / preview cleanly locally.
• Lint clean.
• Examples copy-paste runnable against a local docker-compose
  Postgres (the docker-compose Phase 4 child provides the service).