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/prisma/schema.prisma

@@ -221,6 +221,7 @@ model User {
   knowledgeEntryVersions KnowledgeEntryVersion[] @relation("EntryVersionAuthor")
   llmProviders           LlmProviderInstance[]   @relation("UserLlmProviders")
   federatedIdentities    FederatedIdentity[]
+  llmUsageLogs           LlmUsageLog[]           @relation("UserLlmUsageLogs")
 
   @@map("users")
 }
@@ -272,6 +273,7 @@ model Workspace {
   federationConnections         FederationConnection[]
   federationMessages            FederationMessage[]
   federationEventSubscriptions  FederationEventSubscription[]
+  llmUsageLogs                  LlmUsageLog[]
 
   @@index([ownerId])
   @@map("workspaces")
@@ -1036,6 +1038,7 @@ model LlmProviderInstance {
   user                 User?                  @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
   personalities        Personality[]          @relation("PersonalityLlmProvider")
   workspaceLlmSettings WorkspaceLlmSettings[] @relation("WorkspaceLlmProvider")
+  llmUsageLogs         LlmUsageLog[]          @relation("LlmUsageLogs")
 
   @@index([userId])
   @@index([providerType])
@@ -1383,3 +1386,53 @@ model FederationEventSubscription {
   @@index([workspaceId, isActive])
   @@map("federation_event_subscriptions")
 }
+
+// ============================================
+// LLM USAGE TRACKING MODULE
+// ============================================
+
+model LlmUsageLog {
+  id          String @id @default(uuid()) @db.Uuid
+  workspaceId String @map("workspace_id") @db.Uuid
+  userId      String @map("user_id") @db.Uuid
+
+  // LLM provider and model info
+  provider            String  @db.VarChar(50)
+  model               String  @db.VarChar(100)
+  providerInstanceId  String? @map("provider_instance_id") @db.Uuid
+
+  // Token usage
+  promptTokens     Int @default(0) @map("prompt_tokens")
+  completionTokens Int @default(0) @map("completion_tokens")
+  totalTokens      Int @default(0) @map("total_tokens")
+
+  // Optional cost (in cents for precision)
+  costCents Float? @map("cost_cents")
+
+  // Task type for routing analytics
+  taskType String? @map("task_type") @db.VarChar(50)
+
+  // Optional reference to conversation/session
+  conversationId String? @map("conversation_id") @db.Uuid
+
+  // Duration in milliseconds
+  durationMs Int? @map("duration_ms")
+
+  // Timestamp
+  createdAt DateTime @default(now()) @map("created_at") @db.Timestamptz
+
+  // Relations
+  workspace              Workspace            @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+  user                   User                 @relation("UserLlmUsageLogs", fields: [userId], references: [id], onDelete: Cascade)
+  llmProviderInstance    LlmProviderInstance? @relation("LlmUsageLogs", fields: [providerInstanceId], references: [id], onDelete: SetNull)
+
+  @@index([workspaceId])
+  @@index([workspaceId, createdAt])
+  @@index([userId])
+  @@index([provider])
+  @@index([model])
+  @@index([providerInstanceId])
+  @@index([taskType])
+  @@index([conversationId])
+  @@map("llm_usage_logs")
+}