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

homelab/runner/docker-compose.yml

@@ -1,7 +1,7 @@
 # LibNovel homelab runner
 #
 # Connects to production PocketBase and MinIO via public subdomains.
-# All secrets come from Doppler (project=libnovel, config=prd).
+# All secrets come from Doppler (project=libnovel, config=prd_homelab).
 # Run with: doppler run -- docker compose up -d
 #
 # Differs from prod runner:
@@ -11,12 +11,31 @@
 #   - MEILI_URL               → https://search.libnovel.cc (Caddy-proxied)
 #   - VALKEY_ADDR             → unset (not exposed publicly)
 #   - RUNNER_SKIP_INITIAL_CATALOGUE_REFRESH=true
+#   - Redis service for Asynq task queue (local to homelab, exposed to prod via Caddy TCP proxy)
 
 services:
+  redis:
+    image: redis:7-alpine
+    restart: unless-stopped
+    volumes:
+      - redis_data:/data
+    command: >
+      redis-server
+      --appendonly yes
+      --requirepass "${REDIS_PASSWORD}"
+    healthcheck:
+      test: ["CMD", "redis-cli", "-a", "${REDIS_PASSWORD}", "ping"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
   runner:
     image: kalekber/libnovel-runner:latest
     restart: unless-stopped
     stop_grace_period: 135s
+    depends_on:
+      redis:
+        condition: service_healthy
     environment:
       # ── PocketBase ──────────────────────────────────────────────────────────
       POCKETBASE_URL: "https://pb.libnovel.cc"
@@ -42,6 +61,14 @@ services:
       KOKORO_URL: "${KOKORO_URL}"
       KOKORO_VOICE: "${KOKORO_VOICE}"
 
+      # ── Pocket TTS ──────────────────────────────────────────────────────────
+      POCKET_TTS_URL: "${POCKET_TTS_URL}"
+
+      # ── Asynq / Redis (local service) ───────────────────────────────────────
+      # The runner connects to the local Redis sidecar.
+      REDIS_ADDR: "redis:6379"
+      REDIS_PASSWORD: "${REDIS_PASSWORD}"
+
       # ── Runner tuning ───────────────────────────────────────────────────────
       RUNNER_WORKER_ID: "${RUNNER_WORKER_ID}"
       RUNNER_POLL_INTERVAL: "${RUNNER_POLL_INTERVAL}"
@@ -60,3 +87,6 @@ services:
       interval: 60s
       timeout: 5s
       retries: 3
+
+volumes:
+  redis_data: