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)

frontend/src/pages/MapPage.tsx

@@ -24,6 +24,7 @@ import {
   Text,
   Toolbar,
   ToolbarButton,
+  Tooltip,
   makeStyles,
   tokens,
 } from "@fluentui/react-components";
@@ -286,7 +287,18 @@ export function MapPage(): React.JSX.Element {
                     </TableCell>
                     <TableCell>
                       <TableCellLayout>
-                        {ban.country_name ?? ban.country_code ?? "—"}
+                        {ban.country_name ?? ban.country_code ? (
+                          ban.country_name ?? ban.country_code
+                        ) : (
+                          <Tooltip
+                            content="Country could not be resolved — will retry automatically."
+                            relationship="description"
+                          >
+                            <Text style={{ color: tokens.colorNeutralForeground3 }}>
+                              —
+                            </Text>
+                          </Tooltip>
+                        )}
                       </TableCellLayout>
                     </TableCell>
                     <TableCell>