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/app.module.ts

@@ -22,6 +22,7 @@ import { KnowledgeModule } from "./knowledge/knowledge.module";
 import { UsersModule } from "./users/users.module";
 import { WebSocketModule } from "./websocket/websocket.module";
 import { LlmModule } from "./llm/llm.module";
+import { LlmUsageModule } from "./llm-usage/llm-usage.module";
 import { BrainModule } from "./brain/brain.module";
 import { CronModule } from "./cron/cron.module";
 import { AgentTasksModule } from "./agent-tasks/agent-tasks.module";
@@ -80,6 +81,7 @@ import { FederationModule } from "./federation/federation.module";
     UsersModule,
     WebSocketModule,
     LlmModule,
+    LlmUsageModule,
     BrainModule,
     CronModule,
     AgentTasksModule,