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/telemetry/span-context.service.ts

@@ -0,0 +1,73 @@
+import { Injectable } from "@nestjs/common";
+import { context, trace, type Span, type Context } from "@opentelemetry/api";
+
+/**
+ * Service for managing OpenTelemetry span context propagation.
+ * Provides utilities for accessing and manipulating the active trace context.
+ *
+ * @example
+ * ```typescript
+ * const activeSpan = spanContextService.getActiveSpan();
+ * if (activeSpan) {
+ *   activeSpan.setAttribute('custom.key', 'value');
+ * }
+ * ```
+ */
+@Injectable()
+export class SpanContextService {
+  /**
+   * Get the currently active span from the context.
+   *
+   * @returns The active span, or undefined if no span is active
+   */
+  getActiveSpan(): Span | undefined {
+    return trace.getActiveSpan();
+  }
+
+  /**
+   * Get the current trace context.
+   *
+   * @returns The current context
+   */
+  getContext(): Context {
+    return context.active();
+  }
+
+  /**
+   * Execute a function within a specific context.
+   *
+   * @param ctx - The context to run the function in
+   * @param fn - The function to execute
+   * @returns The result of the function
+   *
+   * @example
+   * ```typescript
+   * const result = spanContextService.with(customContext, () => {
+   *   // This code runs with customContext active
+   *   return doSomething();
+   * });
+   * ```
+   */
+  with<T>(ctx: Context, fn: () => T): T {
+    return context.with(ctx, fn);
+  }
+
+  /**
+   * Set a span as active for the duration of a function execution.
+   *
+   * @param span - The span to make active
+   * @param fn - The function to execute with the active span
+   * @returns The result of the function
+   *
+   * @example
+   * ```typescript
+   * const result = await spanContextService.withActiveSpan(span, async () => {
+   *   // This code runs with span active
+   *   return await doAsyncWork();
+   * });
+   * ```
+   */
+  withActiveSpan<T>(span: Span, fn: () => T): T {
+    return context.with(trace.setSpan(context.active(), span), fn);
+  }
+}