feat: MCP Client plugin (Task 5.17) - integrate external MCP servers for AI agents

- New plugin app/plugins/builtins/mcp_client/ with server config CRUD
- Models: McpServerConfig with TenantMixin (name, url, api_token, enabled)
- Routes: GET/POST/PATCH/DELETE /api/v1/mcp-client/servers
- Routes: GET /servers/{id}/tools, POST /servers/{id}/execute
- client.py: async MCP client using httpx for external server calls
- tool_registry_integration.py: registers external MCP tools in AI tool registry
- Migration: 0001_initial.sql for mcp_server_configs table
- Frontend: mcpClient.ts API client with React Query hooks
- Frontend: MCP Client settings UI in SettingsMcp.tsx (server CRUD, tool viewing)
- Tests: 8 tests covering CRUD, auth, tool registry integration

app/plugins/builtins/mcp_client/__init__.py

@@ -0,0 +1,5 @@
+"""MCP Client builtin plugin."""
+
+from app.plugins.builtins.mcp_client.plugin import McpClientPlugin
+
+__all__ = ["McpClientPlugin"]