Code formatting:
- Run ruff format on all 40 files (style consistency)
- ruff check --fix: remove unused imports and variables in tests

.gitlab-ci.yml:
- lint: add `ruff format --check` + env vars for import smoke test
- build: add `tags: [vps-runner]`, remove docker:dind (shell executor
  uses host Docker); retag image:latest → image:previous before each
  build for one-step rollback
- deploy: add `tags: [vps-runner]`, remove SSH setup (runs directly on
  VPS); add pg_dump backup step (keeps 10 latest); poll container health
  instead of sleep 5; add `alembic upgrade head` after container is up

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>