Add async lock protection to geo service cache and mark Task 16 done

backend/tests/test_services/test_ban_service.py

@@ -700,7 +700,7 @@ class TestOriginFilter:
         assert len(result.bans) == 205
         assert all(b.country_code == "DE" for b in result.bans)
 
-        geo_service.clear_cache()
+        await geo_service.clear_cache()
 
     async def test_bans_by_country_source_archive_reads_archive(
         self, app_db_with_archive: aiosqlite.Connection
@@ -769,7 +769,7 @@ class TestBansbyCountryBackground:
         assert result.total == 3
         # Country counts should reflect the cached data.
         assert "DE" in result.countries or "US" in result.countries or "JP" in result.countries
-        geo_service.clear_cache()
+        await geo_service.clear_cache()
 
     async def test_uncached_ips_trigger_background_task(
         self, mixed_origin_db_path: str
@@ -778,7 +778,7 @@ class TestBansbyCountryBackground:
         resolution and the response returns without blocking."""
         from app.services import geo_service
 
-        geo_service.clear_cache()  # ensure cache is empty
+        await geo_service.clear_cache()  # ensure cache is empty
 
         with (
             patch(
@@ -810,7 +810,7 @@ class TestBansbyCountryBackground:
         """When http_session is None, no background task is created."""
         from app.services import geo_service
 
-        geo_service.clear_cache()
+        await geo_service.clear_cache()
 
         with (
             patch(