feat(#309): Add LLM usage tracking and analytics

Implements comprehensive LLM usage tracking with analytics endpoints.

Implementation:
- Added LlmUsageLog model to Prisma schema
- Created llm-usage module with service, controller, and DTOs
- Added tracking for token usage, costs, and durations
- Implemented analytics aggregation by provider, model, and task type
- Added filtering by workspace, provider, model, user, and date range

Testing:
- 20 unit tests with 90.8% coverage (exceeds 85% requirement)
- Tests for service and controller with full error handling
- Tests use Vitest following project conventions

API Endpoints:
- GET /api/llm-usage/analytics - Aggregated usage analytics
- GET /api/llm-usage/by-workspace/:workspaceId - Workspace usage logs
- GET /api/llm-usage/by-workspace/:workspaceId/provider/:provider - Provider logs
- GET /api/llm-usage/by-workspace/:workspaceId/model/:model - Model logs

Database:
- LlmUsageLog table with indexes for efficient queries
- Relations to User, Workspace, and LlmProviderInstance
- Ready for migration with: pnpm prisma migrate dev

Refs #309

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

apps/api/src/llm-usage/llm-usage.controller.ts

@@ -0,0 +1,68 @@
+import { Controller, Get, Param, Query } from "@nestjs/common";
+import { LlmUsageService } from "./llm-usage.service";
+import type { UsageAnalyticsQueryDto } from "./dto";
+
+/**
+ * LLM Usage Controller
+ *
+ * Provides endpoints for querying LLM usage analytics and logs.
+ * All endpoints return data in the standard API response format.
+ */
+@Controller("llm-usage")
+export class LlmUsageController {
+  constructor(private readonly llmUsageService: LlmUsageService) {}
+
+  /**
+   * Get aggregated usage analytics.
+   * Supports filtering by workspace, provider, model, user, and date range.
+   *
+   * @param query - Analytics query parameters
+   * @returns Aggregated usage analytics
+   */
+  @Get("analytics")
+  async getAnalytics(@Query() query: UsageAnalyticsQueryDto) {
+    const data = await this.llmUsageService.getUsageAnalytics(query);
+    return { data };
+  }
+
+  /**
+   * Get all usage logs for a specific workspace.
+   *
+   * @param workspaceId - Workspace UUID
+   * @returns Array of usage logs
+   */
+  @Get("by-workspace/:workspaceId")
+  async getUsageByWorkspace(@Param("workspaceId") workspaceId: string) {
+    const data = await this.llmUsageService.getUsageByWorkspace(workspaceId);
+    return { data };
+  }
+
+  /**
+   * Get usage logs for a specific provider within a workspace.
+   *
+   * @param workspaceId - Workspace UUID
+   * @param provider - Provider name
+   * @returns Array of usage logs
+   */
+  @Get("by-workspace/:workspaceId/provider/:provider")
+  async getUsageByProvider(
+    @Param("workspaceId") workspaceId: string,
+    @Param("provider") provider: string
+  ) {
+    const data = await this.llmUsageService.getUsageByProvider(workspaceId, provider);
+    return { data };
+  }
+
+  /**
+   * Get usage logs for a specific model within a workspace.
+   *
+   * @param workspaceId - Workspace UUID
+   * @param model - Model name
+   * @returns Array of usage logs
+   */
+  @Get("by-workspace/:workspaceId/model/:model")
+  async getUsageByModel(@Param("workspaceId") workspaceId: string, @Param("model") model: string) {
+    const data = await this.llmUsageService.getUsageByModel(workspaceId, model);
+    return { data };
+  }
+}