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/config.py

@@ -45,6 +45,13 @@ class Settings(BaseSettings):
         default="info",
         description="Application log level: debug | info | warning | error | critical.",
     )
+    geoip_db_path: str | None = Field(
+        default=None,
+        description=(
+            "Optional path to a MaxMind GeoLite2-Country .mmdb file. "
+            "When set, failed ip-api.com lookups fall back to local resolution."
+        ),
+    )
 
     model_config = SettingsConfigDict(
         env_prefix="BANGUI_",