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/internal/api/routes.go

@@ -57,6 +57,9 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 	// Initialize agent handler (sqlc-backed)
 	agentHandler := NewNodeAgentHandler(db)
 
+	// Start the cron scheduler — executes due cron_jobs via docker exec.
+	StartCronScheduler(context.Background(), db, dockerClient)
+
 	// Initialize database handler
 	databaseHandler := NewDatabaseHandler(db.DB, dockerClient)