Fix geo cache write performance: batch commits, read-only GETs, dirty flush

- Remove per-IP db.commit() from _persist_entry() and _persist_neg_entry();
  add a single commit after the full lookup_batch() chunk loop instead.
  Reduces commits from ~5,200 to 1 per bans/by-country request.

- Remove db dependency from GET /api/dashboard/bans and
  GET /api/dashboard/bans/by-country; pass app_db=None so no SQLite
  writes occur during read-only requests.

- Add _dirty set to geo_service; _store() marks resolved IPs dirty.
  New flush_dirty(db) batch-upserts all dirty entries in one transaction.
  New geo_cache_flush APScheduler task flushes every 60 s so geo data
  is persisted without blocking requests.

backend/app/main.py

@@ -34,7 +34,7 @@ from starlette.middleware.base import BaseHTTPMiddleware
 from app.config import Settings, get_settings
 from app.db import init_db
 from app.routers import auth, bans, blocklist, config, dashboard, geo, health, history, jails, server, setup
-from app.tasks import blocklist_import, health_check
+from app.tasks import blocklist_import, geo_cache_flush, health_check
 from app.utils.fail2ban_client import Fail2BanConnectionError, Fail2BanProtocolError
 
 # ---------------------------------------------------------------------------
@@ -151,6 +151,9 @@ async def _lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     # --- Blocklist import scheduled task ---
     blocklist_import.register(app)
 
+    # --- Periodic geo cache flush to SQLite ---
+    geo_cache_flush.register(app)
+
     log.info("bangui_started")
 
     try: