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).

homelab/docker-compose.yml

@@ -72,9 +72,9 @@ services:
       LOG_LEVEL: "${LOG_LEVEL}"
       GLITCHTIP_DSN: "${GLITCHTIP_DSN}"
 
-      # OTel — send runner traces/metrics to the local collector
-      OTEL_EXPORTER_OTLP_ENDPOINT: "http://otel-collector:4317"
-      OTEL_SERVICE_NAME: "libnovel-runner"
+      # OTel — send runner traces/metrics to the local collector (HTTP)
+      OTEL_EXPORTER_OTLP_ENDPOINT: "http://otel-collector:4318"
+      OTEL_SERVICE_NAME: "runner"
 
     healthcheck:
       test: ["CMD", "/healthcheck", "file", "/tmp/runner.alive", "120"]