refactor: improve backend type safety and import organization

- Add TYPE_CHECKING guards for runtime-expensive imports (aiohttp, aiosqlite)
- Reorganize imports to follow PEP 8 conventions
- Convert TypeAlias to modern PEP 695 type syntax (where appropriate)
- Use Sequence/Mapping from collections.abc for type hints (covariant)
- Replace string literals with cast() for improved type inference
- Fix casting of Fail2BanResponse and TypedDict patterns
- Add IpLookupResult TypedDict for precise return type annotation
- Reformat overlong lines for readability (120 char limit)
- Add asyncio_mode and filterwarnings to pytest config
- Update test fixtures with improved type hints

This improves mypy type checking and makes type relationships explicit.

backend/app/tasks/geo_re_resolve.py

@@ -35,7 +35,7 @@ GEO_RE_RESOLVE_INTERVAL: int = 600
 JOB_ID: str = "geo_re_resolve"
 
 
-async def _run_re_resolve(app: "FastAPI") -> None:
+async def _run_re_resolve(app: FastAPI) -> None:
     """Query NULL-country IPs from the database and re-resolve them.
 
     Reads shared resources from ``app.state`` and delegates to

backend/app/tasks/health_check.py

@@ -47,7 +47,7 @@ HEALTH_CHECK_INTERVAL: int = 30
 _ACTIVATION_CRASH_WINDOW: int = 60
 
 
-async def _run_probe(app: "FastAPI") -> None:
+async def _run_probe(app: FastAPI) -> None:
     """Probe fail2ban and cache the result on *app.state*.
 
     Detects online/offline state transitions.  When fail2ban goes offline