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/plugin.py

@@ -0,0 +1,32 @@
+"""MCP Client plugin — allows LeoCRM agents to use external MCP servers."""
+
+from __future__ import annotations
+
+from app.plugins.base import BasePlugin
+from app.plugins.manifest import PluginManifest, PluginRouteDef
+
+
+class McpClientPlugin(BasePlugin):
+    """MCP Client plugin: integrates external MCP servers into the AI tool registry."""
+
+    manifest = PluginManifest(
+        name="mcp_client",
+        version="1.0.0",
+        display_name="MCP Client",
+        description="Allows LeoCRM AI agents to use external MCP servers (web search, code execution, etc.).",
+        dependencies=["permissions"],
+        routes=[
+            PluginRouteDef(
+                path="/api/v1/mcp-client",
+                module="app.plugins.builtins.mcp_client.routes",
+                router_attr="router",
+            ),
+        ],
+        events=[],
+        migrations=["0001_initial.sql"],
+        permissions=[
+            "mcp-client:read",
+            "mcp-client:write",
+            "mcp-client:admin",
+        ],
+    )