feat(#131): add OpenTelemetry tracing infrastructure

Implement comprehensive distributed tracing for HTTP requests and LLM
operations using OpenTelemetry with GenAI semantic conventions.

Features:
- TelemetryService: SDK initialization with OTLP HTTP exporter
- TelemetryInterceptor: Automatic HTTP request spans
- @TraceLlmCall decorator: LLM operation tracing
- GenAI semantic conventions for model/token tracking
- Graceful degradation when tracing disabled

Instrumented:
- All HTTP requests (automatic spans)
- OllamaProvider chat/chatStream/embed operations
- Token counts, model names, durations

Environment:
- OTEL_ENABLED (default: true)
- OTEL_SERVICE_NAME (default: mosaic-api)
- OTEL_EXPORTER_OTLP_ENDPOINT (default: localhost:4318)

Tests: 23 passing with full coverage

Fixes #131

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/package.json

@@ -31,6 +31,13 @@
     "@nestjs/platform-express": "^11.1.12",
     "@nestjs/platform-socket.io": "^11.1.12",
     "@nestjs/websockets": "^11.1.12",
+    "@opentelemetry/api": "^1.9.0",
+    "@opentelemetry/auto-instrumentations-node": "^0.55.0",
+    "@opentelemetry/exporter-trace-otlp-http": "^0.56.0",
+    "@opentelemetry/instrumentation-nestjs-core": "^0.44.0",
+    "@opentelemetry/resources": "^1.30.1",
+    "@opentelemetry/sdk-node": "^0.56.0",
+    "@opentelemetry/semantic-conventions": "^1.28.0",
     "@prisma/client": "^6.19.2",
     "@types/marked": "^6.0.0",
     "@types/multer": "^2.0.0",