Fix startup crash caused by top-level geoip2 import

geoip2 is an optional dependency used only when a MaxMind mmdb path is
configured. Importing it unconditionally at module level caused the server
to crash on startup with ModuleNotFoundError when the package was absent
from the environment.

Move the imports under TYPE_CHECKING (for static analysis) and add lazy
local imports inside init_geoip() and _geoip_lookup() where geoip2 is
actually needed at runtime. The server now starts normally without a
MaxMind database, and geoip2 is loaded on demand if the feature is used.

backend/app/services/geo_service.py

@@ -42,13 +42,13 @@ import time
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
-import geoip2.database
-import geoip2.errors
 import structlog
 
 if TYPE_CHECKING:
     import aiohttp
     import aiosqlite
+    import geoip2.database
+    import geoip2.errors
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -154,6 +154,8 @@ def init_geoip(mmdb_path: str | None) -> None:
         return
     from pathlib import Path  # noqa: PLC0415
 
+    import geoip2.database  # noqa: PLC0415
+
     if not Path(mmdb_path).is_file():
         log.warning("geoip_mmdb_not_found", path=mmdb_path)
         return
@@ -176,6 +178,8 @@ def _geoip_lookup(ip: str) -> GeoInfo | None:
     """
     if _geoip_reader is None:
         return None
+    import geoip2.errors  # noqa: PLC0415
+
     try:
         response = _geoip_reader.country(ip)
         code: str | None = response.country.iso_code or None