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/internal/database/sqlcdb/models.go

@@ -98,6 +98,34 @@ type ContainerInstance struct {
 	UpdatedAt     sql.NullTime          `json:"updated_at"`
 }
 
+type CronExecution struct {
+	ID         uuid.UUID      `json:"id"`
+	CronJobID  uuid.UUID      `json:"cron_job_id"`
+	StartedAt  time.Time      `json:"started_at"`
+	FinishedAt sql.NullTime   `json:"finished_at"`
+	Status     sql.NullString `json:"status"`
+	Output     sql.NullString `json:"output"`
+	Error      sql.NullString `json:"error"`
+}
+
+type CronJob struct {
+	ID         uuid.UUID      `json:"id"`
+	ProjectID  uuid.UUID      `json:"project_id"`
+	ServiceID  uuid.UUID      `json:"service_id"`
+	Name       string         `json:"name"`
+	Schedule   string         `json:"schedule"`
+	Command    string         `json:"command"`
+	Timezone   sql.NullString `json:"timezone"`
+	Enabled    sql.NullBool   `json:"enabled"`
+	LastRunAt  sql.NullTime   `json:"last_run_at"`
+	NextRunAt  sql.NullTime   `json:"next_run_at"`
+	LastStatus sql.NullString `json:"last_status"`
+	LastOutput sql.NullString `json:"last_output"`
+	Retention  sql.NullInt32  `json:"retention"`
+	CreatedAt  sql.NullTime   `json:"created_at"`
+	UpdatedAt  sql.NullTime   `json:"updated_at"`
+}
+
 type DatabaseBackup struct {
 	ID          string         `json:"id"`
 	DatabaseID  string         `json:"database_id"`