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/mcp_servers.yaml

@@ -0,0 +1,60 @@
+# MCP Server Configuration
+#
+# This file defines the MCP servers that the Syndarix backend connects to.
+# Environment variables can be used with ${VAR:-default} syntax.
+#
+# Example:
+#   url: ${MY_SERVER_URL:-http://localhost:8001}
+#
+# For development, these servers typically run as separate Docker containers.
+# See docker-compose.yml for container definitions.
+
+mcp_servers:
+  # LLM Gateway - Multi-provider AI interactions
+  llm-gateway:
+    url: ${LLM_GATEWAY_URL:-http://localhost:8001}
+    transport: http
+    timeout: 60
+    retry_attempts: 3
+    retry_delay: 1.0
+    retry_max_delay: 30.0
+    circuit_breaker_threshold: 5
+    circuit_breaker_timeout: 30.0
+    enabled: true
+    description: "LLM Gateway for Anthropic, OpenAI, Ollama, and other providers"
+
+  # Knowledge Base - RAG and document retrieval
+  knowledge-base:
+    url: ${KNOWLEDGE_BASE_URL:-http://localhost:8002}
+    transport: http
+    timeout: 30
+    retry_attempts: 3
+    circuit_breaker_threshold: 5
+    enabled: true
+    description: "Knowledge Base with pgvector for semantic search and RAG"
+
+  # Git Operations - Repository management
+  git-ops:
+    url: ${GIT_OPS_URL:-http://localhost:8003}
+    transport: http
+    timeout: 120
+    retry_attempts: 2
+    circuit_breaker_threshold: 3
+    enabled: true
+    description: "Git Operations for clone, commit, push, and repository management"
+
+  # Issues - Issue tracker integration
+  issues:
+    url: ${ISSUES_URL:-http://localhost:8004}
+    transport: http
+    timeout: 30
+    retry_attempts: 3
+    circuit_breaker_threshold: 5
+    enabled: true
+    description: "Issue Tracker integration for Gitea, GitHub, and GitLab"
+
+# Global defaults
+default_timeout: 30
+default_retry_attempts: 3
+connection_pool_size: 10
+health_check_interval: 30