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_worker.md

@@ -0,0 +1,49 @@
+---
+name: job_worker
+kind: function
+lang: go
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "func JobWorker(ctx context.Context, q *JobQueue, handler JobHandler, opts ...WorkerOption) error"
+description: "Ejecuta un poll loop bloqueante que desencola y procesa jobs hasta que el context sea cancelado. El handler determina complete vs fail. Opciones: WithPollInterval (default 1s), WithJobTypes."
+tags: [job, queue, worker, goroutine, poll, async, background, infra, concurrency]
+uses_functions: [job_dequeue_go_infra, job_complete_go_infra, job_fail_go_infra]
+uses_types: [job_queue_go_infra, job_go_infra]
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: [context, fmt, time]
+params:
+  - name: ctx
+    desc: "context que al cancelarse detiene el worker limpiamente (graceful shutdown)"
+  - name: q
+    desc: "cola de jobs creada con JobQueueCreate"
+  - name: handler
+    desc: "funcion que procesa un job; retorno nil = completado, error = fallo"
+  - name: opts
+    desc: "opciones: WithPollInterval(d), WithJobTypes(types...)"
+output: "ctx.Err() cuando el context se cancela, o error de configuracion"
+tested: true
+tests:
+  - "worker_graceful_shutdown"
+  - "worker_processes_jobs"
+test_file_path: "functions/infra/job_queue_test.go"
+file_path: "functions/infra/job_worker.go"
+---
+
+## Ejemplo
+
+```go
+ctx, cancel := context.WithCancel(context.Background())
+defer cancel()
+
+go JobWorker(ctx, q, func(job Job) error {
+    fmt.Println("processing", job.ID, job.Type)
+    return nil // complete
+}, WithPollInterval(500*time.Millisecond), WithJobTypes("send_email"))
+```
+
+## Notas
+
+Funcion bloqueante — ejecutar en una goroutine. Retorna `ctx.Err()` cuando el context se cancela (graceful shutdown). Los errores transitorios de dequeue (DB) generan un sleep y retry. El handler tiene la responsabilidad completa del procesamiento; JobWorker se encarga de complete/fail.