feat(backend): implement MCP client infrastructure (#55)

Core MCP client implementation with comprehensive tooling:

**Services:**
- MCPClientManager: Main facade for all MCP operations
- MCPServerRegistry: Thread-safe singleton for server configs
- ConnectionPool: Connection pooling with auto-reconnection
- ToolRouter: Automatic tool routing with circuit breaker
- AsyncCircuitBreaker: Custom async-compatible circuit breaker

**Configuration:**
- YAML-based config with Pydantic models
- Environment variable expansion support
- Transport types: HTTP, SSE, STDIO

**API Endpoints:**
- GET /mcp/servers - List all MCP servers
- GET /mcp/servers/{name}/tools - List server tools
- GET /mcp/tools - List all tools from all servers
- GET /mcp/health - Health check all servers
- POST /mcp/call - Execute tool (admin only)
- GET /mcp/circuit-breakers - Circuit breaker status
- POST /mcp/circuit-breakers/{name}/reset - Reset circuit breaker
- POST /mcp/servers/{name}/reconnect - Force reconnection

**Testing:**
- 156 unit tests with comprehensive coverage
- Tests for all services, routes, and error handling
- Proper mocking and async test support

**Documentation:**
- MCP_CLIENT.md with usage examples
- Phase 2+ workflow documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/api/main.py

@@ -7,6 +7,7 @@ from app.api.routes import (
     auth,
     events,
     issues,
+    mcp,
     oauth,
     oauth_provider,
     organizations,
@@ -31,6 +32,9 @@ api_router.include_router(
 # SSE events router - no prefix, routes define full paths
 api_router.include_router(events.router, tags=["Events"])
 
+# MCP (Model Context Protocol) router
+api_router.include_router(mcp.router, prefix="/mcp", tags=["MCP"])
+
 # Syndarix domain routers
 api_router.include_router(projects.router, prefix="/projects", tags=["Projects"])
 api_router.include_router(