Fix 500 on /info from unresolved get_external_db_name

The info endpoint called externaldb.get_external_db_name(), but the
api-layer externaldb module never re-exported that name from
spoolman.externaldb, so every /info request raised AttributeError ->
HTTP 500. Import the function directly from the core module and call it.

Claude-Session: https://claude.ai/code/session_01VQQtwPBPiMCoZCpBbbqS64

spoolman/api/v1/router.py

@@ -13,6 +13,7 @@ from starlette.responses import Response
 from spoolman import env
 from spoolman.database.database import backup_global_db
 from spoolman.exceptions import ItemNotFoundError
+from spoolman.externaldb import get_external_db_name
 from spoolman.ws import websocket_manager
 
 from . import export, externaldb, field, filament, models, other, search, setting, spool, vendor
@@ -55,7 +56,7 @@ async def info() -> models.Info:
         logs_dir=str(env.get_logs_dir().resolve()),
         backups_dir=str(env.get_backups_dir().resolve()),
         db_type=str(env.get_database_type() or "sqlite"),
-        external_db_name=externaldb.get_external_db_name(),
+        external_db_name=get_external_db_name(),
         git_commit=env.get_commit_hash(),
         build_date=env.get_build_date(),
     )