Fix missing country: neg cache, geoip2 fallback, re-resolve endpoint

- Add 5-min negative cache (_neg_cache) so failing IPs are throttled
  rather than hammering the API on every request
- Add MaxMind GeoLite2 fallback (init_geoip / _geoip_lookup) that fires
  when ip-api fails; controlled by BANGUI_GEOIP_DB_PATH env var
- Fix lookup_batch bug: failed API results were stored in positive cache
- Add _persist_neg_entry: INSERT OR IGNORE into geo_cache with NULL
  country_code so re-resolve can find historically failed IPs
- Add POST /api/geo/re-resolve: clears neg cache, batch-retries all
  geo_cache rows with country_code IS NULL, returns resolved/total count
- BanTable + MapPage: wrap the country — placeholder in a Fluent UI
  Tooltip explaining the retry behaviour
- Add geoip2>=4.8.0 dependency; geoip_db_path config setting
- Tests: add TestNegativeCache (4), TestGeoipFallback (4), TestReResolve (4)

backend/app/main.py

@@ -137,6 +137,7 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     # --- Pre-warm geo cache from the persistent store ---
     from app.services import geo_service  # noqa: PLC0415
 
+    geo_service.init_geoip(settings.geoip_db_path)
     await geo_service.load_cache_from_db(db)
 
     # --- Background task scheduler ---