refactor(db): port cron jobs to sqlc (#80)

Tranche 6 — 15 of 17 raw sites in cron.go. Adds cron_jobs and
cron_executions to schema.sql and a full query set: job CRUD, owner
joins (via project and via service), execution list/insert/finish,
due-job scan for the scheduler, run bookkeeping, and retention trim.

The filtered list query and the sparse UPDATE builder stay raw — both
are genuinely dynamic. Go models keep string IDs; cronJobFrom and
cronExecutionFrom map uuid/null types at the boundary.

TestCronLiveInvariants covers insert/read, both owner joins, the
trigger read, execution lifecycle, retention trim, and delete on live
Postgres.

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app/backend/sqlc/queries/cron.sql

@@ -0,0 +1,58 @@
+-- name: GetCronJobProjectID :one
+SELECT project_id FROM cron_jobs WHERE id = $1;
+
+-- name: GetCronOwnerViaService :one
+SELECT p.owner_id FROM projects p
+JOIN services s ON s.project_id = p.id
+WHERE s.id = $1;
+
+-- name: InsertCronJob :exec
+INSERT INTO cron_jobs (id, project_id, service_id, name, schedule, command, timezone, enabled, next_run_at, retention, created_at, updated_at)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12);
+
+-- name: GetCronJob :one
+SELECT id, project_id, service_id, name, schedule, command, timezone,
+       enabled, last_run_at, next_run_at, last_status, last_output,
+       retention, created_at, updated_at
+FROM cron_jobs WHERE id = $1;
+
+-- name: GetCronJobOwner :one
+SELECT p.owner_id FROM cron_jobs cj
+JOIN projects p ON cj.project_id = p.id
+WHERE cj.id = $1;
+
+-- name: DeleteCronJob :exec
+DELETE FROM cron_jobs WHERE id = $1;
+
+-- name: ListCronExecutions :many
+SELECT id, cron_job_id, started_at, finished_at, status, output, error
+FROM cron_executions
+WHERE cron_job_id = $1
+ORDER BY started_at DESC
+LIMIT 100;
+
+-- name: GetCronJobForTrigger :one
+SELECT cj.service_id, cj.command, cj.schedule, cj.timezone, cj.retention, p.owner_id
+FROM cron_jobs cj
+JOIN projects p ON cj.project_id = p.id
+WHERE cj.id = $1;
+
+-- name: InsertCronExecution :exec
+INSERT INTO cron_executions (id, cron_job_id, started_at, status)
+VALUES ($1, $2, $3, $4);
+
+-- name: ListDueCronJobs :many
+SELECT id, project_id, service_id, name, schedule, command, timezone, enabled, retention
+FROM cron_jobs
+WHERE enabled = TRUE AND next_run_at IS NOT NULL AND next_run_at <= NOW();
+
+-- name: FinishCronExecution :exec
+UPDATE cron_executions SET finished_at = $1, status = $2, output = $3, error = $4 WHERE id = $5;
+
+-- name: UpdateCronJobRun :exec
+UPDATE cron_jobs SET last_run_at = $1, last_status = $2, last_output = $3, next_run_at = $4 WHERE id = $5;
+
+-- name: TrimCronExecutions :exec
+DELETE FROM cron_executions WHERE cron_executions.cron_job_id = $1 AND cron_executions.id NOT IN (
+    SELECT ce.id FROM cron_executions ce WHERE ce.cron_job_id = $1
+    ORDER BY ce.started_at DESC LIMIT $2);