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).

docker-compose.yml

@@ -161,6 +161,8 @@ services:
       KOKORO_URL: "${KOKORO_URL}"
       KOKORO_VOICE: "${KOKORO_VOICE}"
       GLITCHTIP_DSN: "${GLITCHTIP_DSN}"
+      OTEL_EXPORTER_OTLP_ENDPOINT: "${OTEL_EXPORTER_OTLP_ENDPOINT}"
+      OTEL_SERVICE_NAME: "backend"
     healthcheck:
       test: ["CMD", "/healthcheck", "http://localhost:8080/health"]
       interval: 15s
@@ -217,8 +219,9 @@ services:
       KOKORO_URL: "${KOKORO_URL}"
       KOKORO_VOICE: "${KOKORO_VOICE}"
       GLITCHTIP_DSN: "${GLITCHTIP_DSN}"
+      OTEL_EXPORTER_OTLP_ENDPOINT: "${OTEL_EXPORTER_OTLP_ENDPOINT}"
+      OTEL_SERVICE_NAME: "runner"
     healthcheck:
-      # The runner writes /tmp/runner.alive on every poll.
       # 120s = 2× the default 30s poll interval with generous headroom.
       test: ["CMD", "/healthcheck", "file", "/tmp/runner.alive", "120"]
       interval: 60s
@@ -267,6 +270,9 @@ services:
       PUBLIC_UMAMI_SCRIPT_URL: "${PUBLIC_UMAMI_SCRIPT_URL}"
       # GlitchTip client + server-side error tracking
       PUBLIC_GLITCHTIP_DSN: "${PUBLIC_GLITCHTIP_DSN}"
+      # OpenTelemetry tracing
+      OTEL_EXPORTER_OTLP_ENDPOINT: "${OTEL_EXPORTER_OTLP_ENDPOINT}"
+      OTEL_SERVICE_NAME: "ui"
       # OAuth2 providers
       GOOGLE_CLIENT_ID: "${GOOGLE_CLIENT_ID}"
       GOOGLE_CLIENT_SECRET: "${GOOGLE_CLIENT_SECRET}"