fix(database): provide safe default MONGO_URI to prevent CI import crash

- Avoid Empty host error when MONGO_URI is unset during test collection

backend/database.py

@@ -4,7 +4,7 @@ import structlog
 from config import DB_NAME, MONGO_URI, RETENTION_DAYS
 from pymongo import ASCENDING, DESCENDING, TEXT, MongoClient
 
-client = MongoClient(MONGO_URI)
+client = MongoClient(MONGO_URI or "mongodb://localhost:27017")
 db = client[DB_NAME]
 events_collection = db["events"]
 logger = structlog.get_logger("aoc.database")