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_fail.go

@@ -0,0 +1,43 @@
+package infra
+
+import (
+	"fmt"
+	"time"
+)
+
+// JobFail increments the attempt counter and transitions the job to:
+//   - "failed"  when attempts < max_attempts (eligible for retry)
+//   - "dead"    when attempts >= max_attempts (no more retries)
+//
+// errMsg is stored in the error column for debugging.
+func JobFail(q *JobQueue, jobID string, errMsg string) error {
+	if q == nil {
+		return fmt.Errorf("job_fail: queue must not be nil")
+	}
+	if jobID == "" {
+		return fmt.Errorf("job_fail: jobID must not be empty")
+	}
+
+	now := time.Now().UTC().Format(time.RFC3339)
+
+	// Atomically increment attempts and decide status.
+	query := fmt.Sprintf(`
+UPDATE %s
+SET
+    attempts    = attempts + 1,
+    status      = CASE WHEN (attempts + 1) >= max_attempts THEN 'dead' ELSE 'failed' END,
+    error       = ?,
+    completed_at = ?
+WHERE id = ?
+`, q.TableName)
+
+	res, err := q.DB.Exec(query, errMsg, now, jobID)
+	if err != nil {
+		return fmt.Errorf("job_fail: update: %w", err)
+	}
+	n, _ := res.RowsAffected()
+	if n == 0 {
+		return fmt.Errorf("job_fail: job %q not found", jobID)
+	}
+	return nil
+}