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/config/config.go

@@ -86,6 +86,19 @@ type Valkey struct {
 	Addr string
 }
 
+// Redis holds connection settings for the Asynq task queue Redis instance.
+// This is separate from Valkey (presign cache) — it may point to the same
+// Redis or a dedicated one. An empty Addr falls back to PocketBase polling.
+type Redis struct {
+	// Addr is the host:port (or rediss://... URL) of the Redis instance.
+	// Use rediss:// scheme for TLS (e.g. rediss://:password@redis.libnovel.cc:6380).
+	// An empty string disables Asynq and falls back to PocketBase polling.
+	Addr string
+	// Password is the Redis AUTH password.
+	// Not needed when Addr is a full rediss:// URL that includes the password.
+	Password string
+}
+
 // Runner holds settings specific to the runner/worker binary.
 type Runner struct {
 	// PollInterval is how often the runner checks PocketBase for pending tasks.
@@ -125,6 +138,7 @@ type Config struct {
 	Runner      Runner
 	Meilisearch Meilisearch
 	Valkey      Valkey
+	Redis       Redis
 	// LogLevel is one of "debug", "info", "warn", "error".
 	LogLevel string
 }
@@ -192,6 +206,11 @@ func Load() Config {
 		Valkey: Valkey{
 			Addr: envOr("VALKEY_ADDR", ""),
 		},
+
+		Redis: Redis{
+			Addr:     envOr("REDIS_ADDR", ""),
+			Password: envOr("REDIS_PASSWORD", ""),
+		},
 	}
 }