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/backend/go.mod

@@ -61,6 +61,7 @@ require (
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/robfig/cron/v3 v3.0.1 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sethvargo/go-retry v0.3.0 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect