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/cmd/runner/main.go

@@ -20,6 +20,7 @@ import (
 	"time"
 
 	"github.com/getsentry/sentry-go"
+	"github.com/libnovel/backend/internal/asynqqueue"
 	"github.com/libnovel/backend/internal/browser"
 	"github.com/libnovel/backend/internal/config"
 	"github.com/libnovel/backend/internal/kokoro"
@@ -29,6 +30,7 @@ import (
 	"github.com/libnovel/backend/internal/pockettts"
 	"github.com/libnovel/backend/internal/runner"
 	"github.com/libnovel/backend/internal/storage"
+	"github.com/libnovel/backend/internal/taskqueue"
 )
 
 // version and commit are set at build time via -ldflags.
@@ -151,9 +153,23 @@ func run() error {
 		MetricsAddr:                 cfg.Runner.MetricsAddr,
 		CatalogueRefreshInterval:    cfg.Runner.CatalogueRefreshInterval,
 		SkipInitialCatalogueRefresh: cfg.Runner.SkipInitialCatalogueRefresh,
+		RedisAddr:                   cfg.Redis.Addr,
+		RedisPassword:               cfg.Redis.Password,
 	}
+
+	// In Asynq mode the Consumer is a thin wrapper: claim/heartbeat/reap are
+	// no-ops, but FinishAudioTask / FinishScrapeTask / FailTask write back to
+	// PocketBase as before.
+	var consumer taskqueue.Consumer = store
+	if cfg.Redis.Addr != "" {
+		log.Info("runner: asynq mode — using Redis for task dispatch", "addr", cfg.Redis.Addr)
+		consumer = asynqqueue.NewConsumer(store)
+	} else {
+		log.Info("runner: poll mode — using PocketBase for task dispatch")
+	}
+
 	deps := runner.Dependencies{
-		Consumer:    store,
+		Consumer:    consumer,
 		BookWriter:  store,
 		BookReader:  store,
 		AudioStore:  store,