Add OTel distributed tracing to backend, ui, and runner

Go backend:
- Add OTel SDK + otelhttp middleware deps (go.mod)
- New internal/otelsetup package: init OTLP/HTTP TracerProvider from env vars
- cmd/backend/main.go: call otelsetup.Init() after logger + ctx setup
- internal/backend/server.go: wrap mux with otelhttp.NewHandler() before
  sentryhttp, so all HTTP spans are recorded

SvelteKit UI:
- Add @opentelemetry/sdk-node, exporter-trace-otlp-http, resources,
  semantic-conventions
- hooks.server.ts: init NodeSDK when OTEL_EXPORTER_OTLP_ENDPOINT is set;
  graceful shutdown on SIGTERM/SIGINT

Config:
- docker-compose.yml: pass OTEL_EXPORTER_OTLP_ENDPOINT + OTEL_SERVICE_NAME
  to backend, runner, and ui services
- homelab/docker-compose.yml: fix runner OTel endpoint to HTTP port 4318
- Doppler prd: OTEL_EXPORTER_OTLP_ENDPOINT=https://otel.libnovel.cc
- Doppler prd_homelab: OTEL_EXPORTER_OTLP_ENDPOINT=http://otel-collector:4318

All services no-op gracefully when the env var is unset (local dev).

backend/internal/backend/server.go

@@ -33,6 +33,7 @@ import (
 	"github.com/libnovel/backend/internal/kokoro"
 	"github.com/libnovel/backend/internal/meili"
 	"github.com/libnovel/backend/internal/taskqueue"
+	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
 )
 
 // Dependencies holds all external services the backend server depends on.
@@ -170,9 +171,17 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("POST /api/progress/{slug}", s.handleSetProgress)
 	mux.HandleFunc("DELETE /api/progress/{slug}", s.handleDeleteProgress)
 
+	// Wrap mux with OTel tracing (no-op when no TracerProvider is set),
+	// then with Sentry for panic recovery and error reporting.
+	var handler http.Handler = mux
+	handler = otelhttp.NewHandler(handler, "libnovel.backend",
+		otelhttp.WithMessageEvents(otelhttp.ReadEvents, otelhttp.WriteEvents),
+	)
+	handler = sentryhttp.New(sentryhttp.Options{Repanic: true}).Handle(handler)
+
 	srv := &http.Server{
 		Addr:         s.cfg.Addr,
-		Handler:      sentryhttp.New(sentryhttp.Options{Repanic: true}).Handle(mux),
+		Handler:      handler,
 		ReadTimeout:  15 * time.Second,
 		WriteTimeout: 60 * time.Second,
 		IdleTimeout:  60 * time.Second,