feat: cola de jobs asincrona basada en SQLite (issue 0013)

Implementa el subsistema completo de background jobs para apps Go en el
dominio infra. 9 funciones + 3 tipos + 17 tests, todos pasando.

- Tipos: Job (product), JobQueue (product), JobStatus (sum) con
  JobHandler, EnqueueOption y WorkerOption usando functional options pattern
- job_queue_create: CREATE TABLE + indices + WAL mode
- job_enqueue: INSERT con UUID (github.com/google/uuid), WithPriority/WithScheduledAt/WithMaxAttempts
- job_dequeue: SELECT+UPDATE atomico en transaccion exclusiva, filtro por jobTypes
- job_complete / job_fail: transiciones de estado; fail → dead cuando attempts >= max_attempts
- job_status_summary: pura, formatea conteo de jobs por estado
- job_worker: poll loop bloqueante, context-cancelable, graceful shutdown
- job_worker_pool: N workers con golang.org/x/sync/errgroup
- job_cleanup: DELETE jobs terminales mas viejos que olderThan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/job_cleanup.go

@@ -0,0 +1,33 @@
+package infra
+
+import (
+	"fmt"
+	"time"
+)
+
+// JobCleanup deletes jobs in terminal states (completed, failed, dead) that are
+// older than olderThan. Returns the number of rows deleted.
+// This is useful for keeping the jobs table small in long-running applications.
+func JobCleanup(q *JobQueue, olderThan time.Duration) (int64, error) {
+	if q == nil {
+		return 0, fmt.Errorf("job_cleanup: queue must not be nil")
+	}
+
+	cutoff := time.Now().UTC().Add(-olderThan).Format(time.RFC3339)
+
+	query := fmt.Sprintf(`
+DELETE FROM %s
+WHERE status IN ('completed', 'failed', 'dead')
+  AND created_at < ?
+`, q.TableName)
+
+	res, err := q.DB.Exec(query, cutoff)
+	if err != nil {
+		return 0, fmt.Errorf("job_cleanup: delete: %w", err)
+	}
+	n, err := res.RowsAffected()
+	if err != nil {
+		return 0, fmt.Errorf("job_cleanup: rows affected: %w", err)
+	}
+	return n, nil
+}