feat(queue): replace PocketBase polling with Asynq + Redis

Introduce a Redis-backed Asynq task queue so the runner consumes TTS
jobs pushed by the backend instead of polling PocketBase.

- backend/internal/asynqqueue: Producer and Consumer wrappers
- backend/internal/runner: AsynqRunner mux, per-instance Prometheus
  registry (fixes duplicate-collector panic in tests), redisConnOpt
- backend/internal/config: REDIS_ADDR / REDIS_PASSWORD env vars
- backend/cmd/{backend,runner}/main.go: wire Redis when env set; fall
  back to legacy poll mode when unset
- Caddyfile: caddy-l4 TCP proxy for redis.libnovel.cc:6380 → homelab
- caddy/Dockerfile: add --with github.com/mholt/caddy-l4
- docker-compose.yml: Caddy exposes 6380, backend/runner get Redis env
- homelab/runner/docker-compose.yml: Redis sidecar, runner depends_on
- homelab/otel/grafana: Grafana dashboards (backend, catalogue, runner)
  and alerting rules / contact-points provisioning

backend/internal/asynqqueue/consumer.go

@@ -0,0 +1,56 @@
+package asynqqueue
+
+import (
+	"context"
+	"time"
+
+	"github.com/libnovel/backend/internal/domain"
+	"github.com/libnovel/backend/internal/taskqueue"
+)
+
+// Consumer wraps the PocketBase-backed Consumer for result write-back only.
+//
+// When using Asynq, the runner no longer polls for work — Asynq delivers
+// tasks via the ServeMux handlers. The only Consumer operations the handlers
+// need are:
+//   - FinishAudioTask / FinishScrapeTask — write result back to PocketBase
+//   - FailTask — mark PocketBase record as failed
+//
+// ClaimNextAudioTask, ClaimNextScrapeTask, HeartbeatTask, and ReapStaleTasks
+// are all no-ops here because Asynq owns those responsibilities.
+type Consumer struct {
+	pb taskqueue.Consumer // underlying PocketBase consumer (for write-back)
+}
+
+// NewConsumer wraps an existing PocketBase Consumer.
+func NewConsumer(pb taskqueue.Consumer) *Consumer {
+	return &Consumer{pb: pb}
+}
+
+// ── Write-back (delegated to PocketBase) ──────────────────────────────────────
+
+func (c *Consumer) FinishScrapeTask(ctx context.Context, id string, result domain.ScrapeResult) error {
+	return c.pb.FinishScrapeTask(ctx, id, result)
+}
+
+func (c *Consumer) FinishAudioTask(ctx context.Context, id string, result domain.AudioResult) error {
+	return c.pb.FinishAudioTask(ctx, id, result)
+}
+
+func (c *Consumer) FailTask(ctx context.Context, id, errMsg string) error {
+	return c.pb.FailTask(ctx, id, errMsg)
+}
+
+// ── No-ops (Asynq owns claiming / heartbeating / reaping) ───────────────────
+
+func (c *Consumer) ClaimNextScrapeTask(_ context.Context, _ string) (domain.ScrapeTask, bool, error) {
+	return domain.ScrapeTask{}, false, nil
+}
+
+func (c *Consumer) ClaimNextAudioTask(_ context.Context, _ string) (domain.AudioTask, bool, error) {
+	return domain.AudioTask{}, false, nil
+}
+
+func (c *Consumer) HeartbeatTask(_ context.Context, _ string) error { return nil }
+
+func (c *Consumer) ReapStaleTasks(_ context.Context, _ time.Duration) (int, error) { return 0, nil }

backend/internal/asynqqueue/producer.go

@@ -0,0 +1,90 @@
+package asynqqueue
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/hibiken/asynq"
+	"github.com/libnovel/backend/internal/taskqueue"
+)
+
+// Producer dual-writes every task: first to PocketBase (via pb, for audit /
+// UI status), then to Redis via Asynq so the runner picks it up immediately.
+type Producer struct {
+	pb     taskqueue.Producer // underlying PocketBase producer
+	client *asynq.Client
+}
+
+// NewProducer wraps an existing PocketBase Producer with Asynq dispatch.
+func NewProducer(pb taskqueue.Producer, redisOpt asynq.RedisConnOpt) *Producer {
+	return &Producer{
+		pb:     pb,
+		client: asynq.NewClient(redisOpt),
+	}
+}
+
+// Close shuts down the underlying Asynq client connection.
+func (p *Producer) Close() error {
+	return p.client.Close()
+}
+
+// CreateScrapeTask creates a PocketBase record then enqueues an Asynq job.
+func (p *Producer) CreateScrapeTask(ctx context.Context, kind, targetURL string, fromChapter, toChapter int) (string, error) {
+	id, err := p.pb.CreateScrapeTask(ctx, kind, targetURL, fromChapter, toChapter)
+	if err != nil {
+		return "", err
+	}
+
+	payload := ScrapePayload{
+		PBTaskID:    id,
+		Kind:        kind,
+		TargetURL:   targetURL,
+		FromChapter: fromChapter,
+		ToChapter:   toChapter,
+	}
+	taskType := TypeScrapeBook
+	if kind == "catalogue" {
+		taskType = TypeScrapeCatalogue
+	}
+	if err := p.enqueue(ctx, taskType, payload); err != nil {
+		// Non-fatal: PB record exists; runner will pick it up on next poll.
+		return id, fmt.Errorf("asynq enqueue scrape (task still in PB): %w", err)
+	}
+	return id, nil
+}
+
+// CreateAudioTask creates a PocketBase record then enqueues an Asynq job.
+func (p *Producer) CreateAudioTask(ctx context.Context, slug string, chapter int, voice string) (string, error) {
+	id, err := p.pb.CreateAudioTask(ctx, slug, chapter, voice)
+	if err != nil {
+		return "", err
+	}
+
+	payload := AudioPayload{
+		PBTaskID: id,
+		Slug:     slug,
+		Chapter:  chapter,
+		Voice:    voice,
+	}
+	if err := p.enqueue(ctx, TypeAudioGenerate, payload); err != nil {
+		return id, fmt.Errorf("asynq enqueue audio (task still in PB): %w", err)
+	}
+	return id, nil
+}
+
+// CancelTask delegates to PocketBase; Asynq jobs may already be running and
+// cannot be reliably cancelled, so we only update the audit record.
+func (p *Producer) CancelTask(ctx context.Context, id string) error {
+	return p.pb.CancelTask(ctx, id)
+}
+
+// enqueue serialises payload and dispatches it to Asynq.
+func (p *Producer) enqueue(_ context.Context, taskType string, payload any) error {
+	b, err := json.Marshal(payload)
+	if err != nil {
+		return fmt.Errorf("marshal payload: %w", err)
+	}
+	_, err = p.client.Enqueue(asynq.NewTask(taskType, b))
+	return err
+}

backend/internal/asynqqueue/tasks.go

@@ -0,0 +1,46 @@
+// Package asynqqueue provides Asynq-backed implementations of the
+// taskqueue.Producer and taskqueue.Consumer interfaces.
+//
+// Architecture:
+//   - Producer: dual-writes — creates a PocketBase record for audit/UI, then
+//     enqueues an Asynq job so the runner picks it up immediately (sub-ms).
+//   - Consumer: thin wrapper used only for result write-back (FinishAudioTask,
+//     FinishScrapeTask, FailTask). ClaimNext*/Heartbeat/Reap are no-ops because
+//     Asynq owns those responsibilities.
+//   - Handlers: asynq.HandlerFunc wrappers that decode job payloads and invoke
+//     the existing runner logic (runScrapeTask / runAudioTask).
+//
+// Fallback: when REDIS_ADDR is empty the caller should use the plain
+// storage.Store (PocketBase-polling) implementation unchanged.
+package asynqqueue
+
+// Queue names — keep all jobs on the default queue for now.
+// Add separate queues (e.g. "audio", "scrape") later if you need priority.
+const QueueDefault = "default"
+
+// Task type constants used for Asynq routing.
+const (
+	TypeAudioGenerate   = "audio:generate"
+	TypeScrapeBook      = "scrape:book"
+	TypeScrapeCatalogue = "scrape:catalogue"
+)
+
+// AudioPayload is the Asynq job payload for audio generation tasks.
+type AudioPayload struct {
+	// PBTaskID is the PocketBase record ID created before enqueueing.
+	// The handler uses it to write results back via Consumer.FinishAudioTask.
+	PBTaskID string `json:"pb_task_id"`
+	Slug     string `json:"slug"`
+	Chapter  int    `json:"chapter"`
+	Voice    string `json:"voice"`
+}
+
+// ScrapePayload is the Asynq job payload for scrape tasks.
+type ScrapePayload struct {
+	// PBTaskID is the PocketBase record ID created before enqueueing.
+	PBTaskID    string `json:"pb_task_id"`
+	Kind        string `json:"kind"`         // "catalogue", "book", or "book_range"
+	TargetURL   string `json:"target_url"`   // empty for catalogue tasks
+	FromChapter int    `json:"from_chapter"` // 0 unless Kind=="book_range"
+	ToChapter   int    `json:"to_chapter"`   // 0 unless Kind=="book_range"
+}