Build real cron engine + service-detail Cron UI

The cron backend was a stub: calculateNextRun returned now+1h for any
input and executeCronJob slept 2s then wrote "success" without running
the command. Now robfig/cron parses and validates schedules (400 on
invalid), a 60s scheduler goroutine executes due enabled jobs, and
commands run via docker exec sh -c inside the service's container with
stdout+stderr captured into cron_executions. Added Client.ExecRun for
attached output capture. UI: Cron tab on the service detail page with
create/edit/enable-toggle, manual Run, and execution history with logs.
Verified e2e against real Postgres + Docker (trigger and scheduler tick
both produce real captured output).

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/src/lib/api-client.ts

@@ -1323,3 +1323,50 @@ export async function createGitWebhook(input: CreateGitWebhookInput): Promise<Gi
   }
   return payload.webhook;
 }
+
+// --- Cron jobs ---
+
+export type CronJobEntity = components['schemas']['CronJob'];
+export type CronExecutionEntity = components['schemas']['CronExecution'];
+export type CreateCronJobInput = components['schemas']['CreateCronJobRequest'];
+export type UpdateCronJobInput = components['schemas']['UpdateCronJobRequest'];
+
+export async function listCronJobs(serviceId: string): Promise<CronJobEntity[]> {
+  const payload = await requestJson<{ cron_jobs?: CronJobEntity[] }>(
+    `/cron-jobs?service_id=${encodeURIComponent(serviceId)}`,
+  );
+  return payload.cron_jobs ?? [];
+}
+
+export async function createCronJob(input: CreateCronJobInput): Promise<CronJobEntity> {
+  const payload = await requestJson<{ cron_job?: CronJobEntity }>('/cron-jobs', {
+    method: 'POST',
+    body: JSON.stringify(input),
+  });
+  if (!payload.cron_job?.id) {
+    throw new ApiError('Create cron job response is invalid', 500);
+  }
+  return payload.cron_job;
+}
+
+export async function updateCronJob(id: string, input: UpdateCronJobInput): Promise<void> {
+  await requestJson(`/cron-jobs/${encodeURIComponent(id)}`, {
+    method: 'PUT',
+    body: JSON.stringify(input),
+  });
+}
+
+export async function deleteCronJob(id: string): Promise<void> {
+  await requestJson(`/cron-jobs/${encodeURIComponent(id)}`, { method: 'DELETE' });
+}
+
+export async function listCronExecutions(id: string): Promise<CronExecutionEntity[]> {
+  const payload = await requestJson<{ executions?: CronExecutionEntity[] }>(
+    `/cron-jobs/${encodeURIComponent(id)}/executions`,
+  );
+  return payload.executions ?? [];
+}
+
+export async function triggerCronJob(id: string): Promise<void> {
+  await requestJson(`/cron-jobs/${encodeURIComponent(id)}/trigger`, { method: 'POST' });
+}