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).

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ROADMAP.md

@@ -262,8 +262,17 @@ All of these have working APIs and no UI. Cheapest feature wins in the repo.
       detail page section for connection info, actions (start/stop/restart),
       backup create/list/restore. Verify against real Postgres/Redis/MySQL
       images.
-- [ ] **Cron jobs** (`/cron-jobs/*`): list/create/edit per project, execution
-      history with logs, manual trigger. Add "Cron" to Add Service.
+- [x] **Cron jobs** (`/cron-jobs/*`): real engine + UI. Backend was a stub
+      (`calculateNextRun` returned now+1h, `executeCronJob` slept 2s and wrote
+      "success" without running anything). Now: `robfig/cron` parses the
+      schedule (validated on create → 400), a 60s scheduler goroutine executes
+      due enabled jobs, and commands run via `docker exec sh -c` inside the
+      service's `containr-<serviceID>-*` container with stdout+stderr captured
+      into `cron_executions` — verified e2e (trigger + scheduler tick, real
+      alpine output). UI: "Cron" tab on the service detail page — list/create/
+      edit/enable-toggle, manual Run, execution history with logs. (Not added
+      to Add Service: jobs exec into an existing service's container, so they
+      live on the service, not as a standalone type.)
 - [ ] **Preview environments** (`/preview-environments/*`): per-PR/per-branch
       preview creation, promote-to-production action, expiry + cleanup job.
       Decide first whether this stays Phase 2 or moves with env support (§2.4).