feat: lint and format tests in CI

Only src/ was covered, which is why it stayed clean while tests/ drifted to 38
findings. migrations/ stays out: its alembic template emits unused imports, so
every generated revision would fail the gate.

AGENTS.md

@@ -70,7 +70,7 @@ Backend (from `backend/`):
 ```bash
 uv run litestar --app-dir src/chitai/ run --reload   # dev server on :8000
 pytest tests/                                        # needs Docker (pytest-databases)
-ruff format src/
+ruff format src/ tests/
 alchemy --config chitai.database.config.config make-migrations
 alchemy --config chitai.database.config.config upgrade
 
@@ -104,10 +104,12 @@ API docs are served by the running backend at `http://localhost:8000/schema/` (S
 - **Migrations are mandatory.** The app runs with `create_all=False`, so a model change without a
   matching Alembic revision will not reach the database.
 - **CI gates formatting, linting, types and tests.** `.gitea/workflows/ci.yml` runs on every push
-  and pull request: `ruff format --check`, `ruff check`, `pytest`, `prettier --check` and
-  `pnpm check` all **block**; only `eslint` reports without failing, and only until the two
-  `{@html}` findings in `TODO.md` are fixed. A `v*` tag additionally builds and publishes both
-  container images — see `docs/ci-release-pipeline.md`.
+  and pull request: `ruff format --check src/ tests/`, `ruff check src/ tests/`, `pytest`,
+  `prettier --check` and `pnpm check` all **block**; only `eslint` reports without failing, and
+  only until the two `{@html}` findings in `TODO.md` are fixed. Ruff covers `src/` and `tests/`
+  but deliberately not `migrations/`, whose alembic template emits imports it does not use. A
+  `v*` tag additionally builds and publishes both container images — see
+  `docs/ci-release-pipeline.md`.
 - **Commit messages** follow `type: summary` — `feat:`, `fix:`, `refactor:`, `chore:`.
 - The three `README.md` files are user-facing. Agent-facing knowledge belongs in the `AGENTS.md`
   files.