feat(#129): add LLM provider admin API endpoints

Implement REST API endpoints for managing LLM provider instances.

Changes:
- Created DTOs for provider CRUD operations (CreateLlmProviderDto, UpdateLlmProviderDto, LlmProviderResponseDto)
- Implemented LlmProviderAdminController with full CRUD endpoints:
  - GET /llm/admin/providers - List all providers
  - GET /llm/admin/providers/:id - Get provider details
  - POST /llm/admin/providers - Create new provider
  - PATCH /llm/admin/providers/:id - Update provider
  - DELETE /llm/admin/providers/:id - Delete provider
  - POST /llm/admin/providers/:id/test - Test connection
  - POST /llm/admin/reload - Reload from database
- Updated llm-manager.service.ts to support OpenAI and Claude providers
- Added comprehensive test suite with 97.95% coverage
- Proper validation, error handling, and type safety

All tests pass. Pre-commit hooks pass.

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

apps/api/src/llm/llm.module.ts

@@ -1,12 +1,13 @@
 import { Module } from "@nestjs/common";
 import { LlmController } from "./llm.controller";
+import { LlmProviderAdminController } from "./llm-provider-admin.controller";
 import { LlmService } from "./llm.service";
 import { LlmManagerService } from "./llm-manager.service";
 import { PrismaModule } from "../prisma/prisma.module";
 
 @Module({
   imports: [PrismaModule],
-  controllers: [LlmController],
+  controllers: [LlmController, LlmProviderAdminController],
   providers: [LlmService, LlmManagerService],
   exports: [LlmService, LlmManagerService],
 })