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/dto/index.ts

@@ -0,0 +1,2 @@
+export * from "./track-usage.dto";
+export * from "./usage-analytics.dto";

apps/api/src/llm-usage/dto/track-usage.dto.ts

@@ -0,0 +1,49 @@
+import { IsString, IsInt, IsOptional, IsNumber, Min, IsUUID } from "class-validator";
+
+export class TrackUsageDto {
+  @IsUUID()
+  workspaceId!: string;
+
+  @IsUUID()
+  userId!: string;
+
+  @IsString()
+  provider!: string;
+
+  @IsString()
+  model!: string;
+
+  @IsOptional()
+  @IsUUID()
+  providerInstanceId?: string;
+
+  @IsInt()
+  @Min(0)
+  promptTokens!: number;
+
+  @IsInt()
+  @Min(0)
+  completionTokens!: number;
+
+  @IsInt()
+  @Min(0)
+  totalTokens!: number;
+
+  @IsOptional()
+  @IsNumber()
+  @Min(0)
+  costCents?: number;
+
+  @IsOptional()
+  @IsString()
+  taskType?: string;
+
+  @IsOptional()
+  @IsUUID()
+  conversationId?: string;
+
+  @IsOptional()
+  @IsInt()
+  @Min(0)
+  durationMs?: number;
+}

apps/api/src/llm-usage/dto/usage-analytics.dto.ts

@@ -0,0 +1,69 @@
+import { IsOptional, IsDateString, IsUUID, IsString } from "class-validator";
+
+export class UsageAnalyticsQueryDto {
+  @IsOptional()
+  @IsUUID()
+  workspaceId?: string;
+
+  @IsOptional()
+  @IsString()
+  provider?: string;
+
+  @IsOptional()
+  @IsString()
+  model?: string;
+
+  @IsOptional()
+  @IsUUID()
+  userId?: string;
+
+  @IsOptional()
+  @IsDateString()
+  startDate?: string;
+
+  @IsOptional()
+  @IsDateString()
+  endDate?: string;
+}
+
+export interface UsageAnalyticsResponseDto {
+  totalCalls: number;
+  totalPromptTokens: number;
+  totalCompletionTokens: number;
+  totalTokens: number;
+  totalCostCents: number;
+  averageDurationMs: number;
+  byProvider: ProviderUsageDto[];
+  byModel: ModelUsageDto[];
+  byTaskType: TaskTypeUsageDto[];
+}
+
+export interface ProviderUsageDto {
+  provider: string;
+  calls: number;
+  promptTokens: number;
+  completionTokens: number;
+  totalTokens: number;
+  costCents: number;
+  averageDurationMs: number;
+}
+
+export interface ModelUsageDto {
+  model: string;
+  calls: number;
+  promptTokens: number;
+  completionTokens: number;
+  totalTokens: number;
+  costCents: number;
+  averageDurationMs: number;
+}
+
+export interface TaskTypeUsageDto {
+  taskType: string;
+  calls: number;
+  promptTokens: number;
+  completionTokens: number;
+  totalTokens: number;
+  costCents: number;
+  averageDurationMs: number;
+}