feat: add conversion_jobs repository

Also configures the MariaDB pool with timezone: 'auto', since the
default 'local' mode sends dates without timezone conversion and
silently broke expires_at comparisons whenever the app host and DB
server clocks differ (caught by the findExpiredJobs test).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/db.js

@@ -10,6 +10,7 @@ export function getPool(config) {
       password: config.db.password,
       database: config.db.database,
       connectionLimit: 10,
+      timezone: 'auto',
     });
   }
   return pool;