fix: use explicit UTC timestamp in PostgreSQL dialect

PostgreSQL's CURRENT_TIMESTAMP returns session-local time, unlike
SYSUTCDATETIME() (SQL Server) and UTC_TIMESTAMP() (MySQL) which
explicitly return UTC. Switch to (NOW() AT TIME ZONE 'UTC') to
keep operation history timestamps consistent across all databases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/services/database/dialects/postgresql-dialect.ts

@@ -26,7 +26,7 @@ export class PostgreSqlDialect implements SqlDialect {
   }
 
   currentTimestamp(): string {
-    return 'CURRENT_TIMESTAMP'
+    return "(NOW() AT TIME ZONE 'UTC')"
   }
 
   upsert(params: {