fix: SQLite column migration + startup init_schema + missing DB_URL warning

- db.py: auto-migrate missing columns (signal_correct, pnl_dkk, etc.) on existing SQLite DBs
- db.py: print warning to stderr if DATABASE_URL not set
- dashboard.py: call init_schema() at startup so columns always exist

dashboard.py

@@ -769,6 +769,8 @@ def main():
     parser.add_argument("--port", type=int, default=int(os.getenv("PORT", 5001)))
     parser.add_argument("--host", default=os.getenv("HOST", "0.0.0.0"))
     args = parser.parse_args()
+    from db import init_schema
+    init_schema()
     print(f"\n  MoneyMaker Dashboard -> http://localhost:{args.port}\n")
     app.run(host=args.host, port=args.port, debug=False)