feat: add test suite, tox, pre-commit, and ruff formatting

- tests/: 107 unit tests across config, cache, docker_auth, storage,
  and FastAPI routes; all passing under pytest-asyncio auto mode
- tox.ini: runs pytest via uvx --with tox-uv tox (py311)
- .pre-commit-config.yaml: ruff lint + ruff-format at v0.15.12
- pyproject.toml: pytest config (asyncio_mode=auto), ruff config
  (line-length=140), tox/pre-commit added to dev extras
- Makefile: test/tox/pre-commit targets via uvx --python 3.11
- Source files reformatted by ruff-format (no logic changes)

.pre-commit-config.yaml

@@ -0,0 +1,7 @@
+repos:
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.15.12
+    hooks:
+      - id: ruff
+        args: [--fix, --exit-non-zero-on-fix]
+      - id: ruff-format