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/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"
+}