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/src/app.module.ts

@@ -1,4 +1,5 @@
 import { Module } from "@nestjs/common";
+import { APP_INTERCEPTOR } from "@nestjs/core";
 import { AppController } from "./app.controller";
 import { AppService } from "./app.service";
 import { PrismaModule } from "./prisma/prisma.module";
@@ -20,9 +21,11 @@ import { BrainModule } from "./brain/brain.module";
 import { CronModule } from "./cron/cron.module";
 import { AgentTasksModule } from "./agent-tasks/agent-tasks.module";
 import { ValkeyModule } from "./valkey/valkey.module";
+import { TelemetryModule, TelemetryInterceptor } from "./telemetry";
 
 @Module({
   imports: [
+    TelemetryModule,
     PrismaModule,
     DatabaseModule,
     ValkeyModule,
@@ -44,6 +47,12 @@ import { ValkeyModule } from "./valkey/valkey.module";
     AgentTasksModule,
   ],
   controllers: [AppController],
-  providers: [AppService],
+  providers: [
+    AppService,
+    {
+      provide: APP_INTERCEPTOR,
+      useClass: TelemetryInterceptor,
+    },
+  ],
 })
 export class AppModule {}