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/pyproject.toml

@@ -53,6 +53,12 @@ dependencies = [
     # Celery for background task processing (Syndarix agent jobs)
     "celery[redis]>=5.4.0",
     "sse-starlette>=3.1.1",
+    # MCP (Model Context Protocol) for AI agent tool integration
+    "mcp>=1.0.0",
+    # Circuit breaker pattern for resilient connections
+    "pybreaker>=1.0.0",
+    # YAML configuration support
+    "pyyaml>=6.0.0",
 ]
 
 # Development dependencies
@@ -151,6 +157,7 @@ unfixable = []
 "app/alembic/env.py" = ["E402", "F403", "F405"]  # Alembic requires specific import order
 "app/alembic/versions/*.py" = ["E402"]  # Migration files have specific structure
 "tests/**/*.py" = ["S101", "N806", "B017", "N817", "S110", "ASYNC251", "RUF043"]  # pytest: asserts, CamelCase fixtures, blind exceptions, try-pass patterns, and async test helpers are intentional
+"app/services/mcp/*.py" = ["ASYNC109", "S311", "RUF022"]  # timeout is config param not asyncio.timeout; random is ok for jitter; __all__ order is intentional for readability
 "app/models/__init__.py" = ["F401"]  # __init__ files re-export modules
 "app/models/base.py" = ["F401"]  # Re-exports Base for use by other models
 "app/utils/test_utils.py" = ["N806"]  # SQLAlchemy session factories use CamelCase convention
@@ -268,6 +275,14 @@ ignore_missing_imports = true
 module = "httpx.*"
 ignore_missing_imports = true
 
+[[tool.mypy.overrides]]
+module = "pybreaker.*"
+ignore_missing_imports = true
+
+[[tool.mypy.overrides]]
+module = "yaml.*"
+ignore_missing_imports = true
+
 # SQLAlchemy ORM models - Column descriptors cause type confusion
 [[tool.mypy.overrides]]
 module = "app.models.*"
@@ -307,6 +322,11 @@ disable_error_code = ["assignment", "arg-type", "attr-defined", "unused-ignore"]
 module = "app.services.oauth_service"
 disable_error_code = ["assignment", "arg-type", "attr-defined"]
 
+# MCP services - circuit breaker and httpx client handling
+[[tool.mypy.overrides]]
+module = "app.services.mcp.*"
+disable_error_code = ["attr-defined", "arg-type"]
+
 # Test utils - Testing patterns
 [[tool.mypy.overrides]]
 module = "app.utils.auth_test_utils"