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_dequeue.md

@@ -0,0 +1,49 @@
+---
+name: job_dequeue
+kind: function
+lang: go
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "func JobDequeue(q *JobQueue, jobTypes []string) (*Job, error)"
+description: "Extrae atomicamente el siguiente job disponible usando SELECT+UPDATE en una transaccion exclusiva. Retorna nil, nil si la cola esta vacia o no hay jobs que cumplan el filtro. jobTypes limita los tipos dequeued; slice vacio significa todos."
+tags: [job, queue, dequeue, atomic, transaction, sqlite, async, background, infra]
+uses_functions: []
+uses_types: [job_queue_go_infra, job_go_infra, job_status_go_infra]
+returns: [job_go_infra]
+returns_optional: true
+error_type: "error_go_core"
+imports: [database/sql, errors, fmt, strings, time]
+params:
+  - name: q
+    desc: "cola de jobs creada con JobQueueCreate"
+  - name: jobTypes
+    desc: "lista de tipos a desencolar; slice vacio o nil = todos los tipos"
+output: "puntero a Job con status=running, o nil si la cola esta vacia"
+tested: true
+tests:
+  - "enqueue_dequeue_atomicidad"
+  - "dequeue_empty_queue_returns_nil"
+  - "dequeue_priority_order"
+  - "dequeue_jobtype_filter"
+  - "dequeue_scheduled_in_future_waits"
+test_file_path: "functions/infra/job_queue_test.go"
+file_path: "functions/infra/job_dequeue.go"
+---
+
+## Ejemplo
+
+```go
+// Desencolar cualquier tipo
+job, err := JobDequeue(q, nil)
+if job == nil {
+    // cola vacia
+}
+
+// Filtrar por tipo
+job, err = JobDequeue(q, []string{"send_email", "send_sms"})
+```
+
+## Notas
+
+Usa `db.Begin()` + `tx.Rollback()` (deferred) + `tx.Commit()`. El SELECT filtra por `status='pending' AND scheduled_at <= now` y ordena por `priority DESC, scheduled_at ASC`. El UPDATE atomico cambia el status a 'running' y setea `started_at`. Safe para multiples workers concurrentes — SQLite serializa la transaccion.