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.module.ts

@@ -0,0 +1,12 @@
+import { Module } from "@nestjs/common";
+import { LlmUsageService } from "./llm-usage.service";
+import { LlmUsageController } from "./llm-usage.controller";
+import { PrismaModule } from "../prisma/prisma.module";
+
+@Module({
+  imports: [PrismaModule],
+  controllers: [LlmUsageController],
+  providers: [LlmUsageService],
+  exports: [LlmUsageService],
+})
+export class LlmUsageModule {}