feat: CI/CD pipeline + sortedcontainers for O(log n) index ops

CI/CD (.gitea/workflows/ci.yml):
- Lint: ruff + mypy on every push/PR
- Test: pytest with coverage report (175 tests)
- Security: bandit SAST + pip-audit dependency scan
- Build: Docker image verification

sortedcontainers (backend/search.py):
- Replace bisect with SortedList for _sorted_tokens
- O(log n) add() / discard() instead of O(n) insort/pop
- SortedList.bisect_left() for prefix search
- Add sortedcontainers>=2.4.0 to requirements.txt

backend/requirements.txt

@@ -8,4 +8,5 @@ aiohttp>=3.9.0
 watchdog>=4.0.0
 argon2-cffi>=23.1.0
 python-jose>=3.3.0
+sortedcontainers>=2.4.0
 weasyprint>=60.0