feat(geo): add cache hit/miss metrics and prewarm support

- Add _hits/_misses counters to GeoCache for cache hit/miss ratio tracking
- Reset counters on clear()
- Count hits before misses in lookup_batch() to avoid interleaving
- Add synchronous prewarm() using asyncio.create_task for fire-and-forget
- Add hits/misses fields to GeoCacheStatsResponse model
- Add TestCacheMetrics (5 tests), TestPrewarm (3 tests), TestLargeBanList (2 tests)
- Fix _make_async_db() mock: db.execute is not async, returns ctx manager
- Move collections.abc to TYPE_CHECKING block (TC003)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/models/geo.py

@@ -75,6 +75,8 @@ class GeoCacheStatsResponse(BanGuiBaseModel):
     unresolved: int = Field(..., description="Number of geo_cache rows with country_code IS NULL.")
     neg_cache_size: int = Field(..., description="Number of entries in the in-memory negative cache.")
     dirty_size: int = Field(..., description="Number of newly resolved entries not yet flushed to disk.")
+    hits: int = Field(default=0, description="Number of cache hits since last clear.")
+    misses: int = Field(default=0, description="Number of cache misses since last clear.")
 
 class GeoReResolveResponse(BanGuiBaseModel):
     """Response for ``POST /api/geo/re-resolve``.