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/services/mcp/__init__.py

@@ -0,0 +1,85 @@
+"""
+MCP Client Service Package
+
+Provides infrastructure for communicating with MCP (Model Context Protocol)
+servers. This is the foundation for AI agent tool integration.
+
+Usage:
+    from app.services.mcp import get_mcp_client, MCPClientManager
+
+    # In FastAPI route
+    async def my_route(mcp: MCPClientManager = Depends(get_mcp_client)):
+        result = await mcp.call_tool("llm-gateway", "chat", {"prompt": "Hello"})
+
+    # Direct usage
+    manager = MCPClientManager()
+    await manager.initialize()
+    result = await manager.call_tool("issues", "create_issue", {...})
+    await manager.shutdown()
+"""
+
+from .client_manager import (
+    MCPClientManager,
+    ServerHealth,
+    get_mcp_client,
+    reset_mcp_client,
+    shutdown_mcp_client,
+)
+from .config import (
+    MCPConfig,
+    MCPServerConfig,
+    TransportType,
+    create_default_config,
+    load_mcp_config,
+)
+from .connection import ConnectionPool, ConnectionState, MCPConnection
+from .exceptions import (
+    MCPCircuitOpenError,
+    MCPConnectionError,
+    MCPError,
+    MCPServerNotFoundError,
+    MCPTimeoutError,
+    MCPToolError,
+    MCPToolNotFoundError,
+    MCPValidationError,
+)
+from .registry import MCPServerRegistry, ServerCapabilities, get_registry
+from .routing import AsyncCircuitBreaker, CircuitState, ToolInfo, ToolResult, ToolRouter
+
+__all__ = [
+    # Main facade
+    "MCPClientManager",
+    "get_mcp_client",
+    "shutdown_mcp_client",
+    "reset_mcp_client",
+    "ServerHealth",
+    # Configuration
+    "MCPConfig",
+    "MCPServerConfig",
+    "TransportType",
+    "load_mcp_config",
+    "create_default_config",
+    # Registry
+    "MCPServerRegistry",
+    "ServerCapabilities",
+    "get_registry",
+    # Connection
+    "ConnectionPool",
+    "ConnectionState",
+    "MCPConnection",
+    # Routing
+    "ToolRouter",
+    "ToolInfo",
+    "ToolResult",
+    "AsyncCircuitBreaker",
+    "CircuitState",
+    # Exceptions
+    "MCPError",
+    "MCPConnectionError",
+    "MCPTimeoutError",
+    "MCPToolError",
+    "MCPServerNotFoundError",
+    "MCPToolNotFoundError",
+    "MCPCircuitOpenError",
+    "MCPValidationError",
+]