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/cmd/backend/main.go

@@ -26,6 +26,7 @@ import (
 	"github.com/libnovel/backend/internal/config"
 	"github.com/libnovel/backend/internal/kokoro"
 	"github.com/libnovel/backend/internal/meili"
+	"github.com/libnovel/backend/internal/otelsetup"
 	"github.com/libnovel/backend/internal/storage"
 )
 
@@ -70,6 +71,16 @@ func run() error {
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
 
+	// ── OpenTelemetry tracing ─────────────────────────────────────────────────
+	otelShutdown, err := otelsetup.Init(ctx, version)
+	if err != nil {
+		return fmt.Errorf("init otel: %w", err)
+	}
+	if otelShutdown != nil {
+		defer otelShutdown()
+		log.Info("otel tracing enabled", "endpoint", os.Getenv("OTEL_EXPORTER_OTLP_ENDPOINT"))
+	}
+
 	// ── Storage ──────────────────────────────────────────────────────────────
 	store, err := storage.NewStore(ctx, cfg, log)
 	if err != nil {