fix: Date picker bug

backend/src/core/config/database.ts

@@ -2,10 +2,15 @@
  * @ai-summary PostgreSQL connection pool configuration
  * @ai-context Shared pool for all feature repositories
  */
-import { Pool } from 'pg';
+import { Pool, types } from 'pg';
 import { logger } from '../logging/logger';
 import { appConfig } from './config-loader';
 
+// Override DATE type parser to return plain YYYY-MM-DD strings instead of Date objects.
+// Default pg behavior creates Date objects at local midnight, which shift dates when
+// serialized to JSON via toISOString() (UTC conversion) for clients in other timezones.
+types.setTypeParser(1082, (val: string) => val);
+
 export const pool = new Pool({
   connectionString: appConfig.getDatabaseUrl(),
   max: 10,