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).

ui/src/hooks.server.ts

@@ -7,6 +7,26 @@ import { env as pubEnv } from '$env/dynamic/public';
 import { log } from '$lib/server/logger';
 import { createUserSession, touchUserSession, isSessionRevoked } from '$lib/server/pocketbase';
 import { drain as drainPresignCache } from '$lib/server/presignCache';
+import { NodeSDK } from '@opentelemetry/sdk-node';
+import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-http';
+import { Resource } from '@opentelemetry/resources';
+import { ATTR_SERVICE_NAME, ATTR_SERVICE_VERSION } from '@opentelemetry/semantic-conventions';
+
+// ─── OpenTelemetry server-side tracing ────────────────────────────────────────
+// No-op when OTEL_EXPORTER_OTLP_ENDPOINT is unset (e.g. local dev).
+const otlpEndpoint = process.env.OTEL_EXPORTER_OTLP_ENDPOINT;
+if (otlpEndpoint) {
+	const sdk = new NodeSDK({
+		resource: new Resource({
+			[ATTR_SERVICE_NAME]: process.env.OTEL_SERVICE_NAME ?? 'ui',
+			[ATTR_SERVICE_VERSION]: pubEnv.PUBLIC_BUILD_VERSION ?? 'dev'
+		}),
+		traceExporter: new OTLPTraceExporter({ url: `${otlpEndpoint}/v1/traces` })
+	});
+	sdk.start();
+	process.once('SIGTERM', () => sdk.shutdown().catch(() => {}));
+	process.once('SIGINT', () => sdk.shutdown().catch(() => {}));
+}
 
 // ─── Sentry / GlitchTip server-side error tracking ────────────────────────────
 // No-op when PUBLIC_GLITCHTIP_DSN is unset (e.g. local dev).