feat(auth): implement Resend email quota and rate limit protection

Track Resend API usage via response headers to prevent quota exhaustion
Block auth requests with 503 when email delivery limits are reached

backend/src/server.ts

@@ -186,6 +186,10 @@ app.setErrorHandler(async (error, _request, reply) => {
     return reply.code(429).send({ message: await serverMessage(locale, 'rateLimited') });
   }
 
+  if (pgError.statusCode === 503) {
+    return reply.code(503).send({ message: await localizedStatusMessage(locale, pgError.message) });
+  }
+
   if (pgError.statusCode && pgError.statusCode < 500) {
     return reply.code(pgError.statusCode).send({ message: await localizedStatusMessage(locale, pgError.message) });
   }