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.sum

@@ -134,6 +134,8 @@ github.com/pressly/goose/v3 v3.24.1 h1:bZmxRco2uy5uu5Ng1MMVEfYsFlrMJI+e/VMXHQ3C4
 github.com/pressly/goose/v3 v3.24.1/go.mod h1:rEWreU9uVtt0DHCyLzF9gRcWiiTF/V+528DV+4DORug=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
+github.com/robfig/cron/v3 v3.0.1 h1:WdRxkvbJztn8LMz/QEvLN5sBU+xKpSqwwUO1Pjr4qDs=
+github.com/robfig/cron/v3 v3.0.1/go.mod h1:eQICP3HwyT7UooqI/z+Ov+PtYAWygg1TEWWzGIFLtro=
 github.com/rogpeppe/go-internal v1.14.1 h1:UQB4HGPB6osV0SQTLymcB4TgvyWu6ZyliaW0tI/otEQ=
 github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7so1lCWt35ZSgc=
 github.com/rs/cors v1.10.1 h1:L0uuZVXIKlI1SShY2nhFfo44TYvDPQ1w4oFkUJNfhyo=