Add unread indication to subscription cards

backend/model.py

@@ -6,7 +6,10 @@ from sqlalchemy.sql.sqltypes import DateTime
 import os
 
 # Initialize the database
-DATABASE_URL = os.getenv("DATABASE_URL", "sqlite:///./project_monitor.db")
+DATABASE_URL = os.getenv("DATABASE_URL")
+
+if not DATABASE_URL:
+    raise ValueError("DATABASE_URL environment variable is not set")
 
 # SQLAlchemy setup
 engine = create_engine(DATABASE_URL, connect_args={"check_same_thread": False})