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/migrations/0001_initial.sql

@@ -0,0 +1,15 @@
+CREATE TABLE IF NOT EXISTS mcp_server_configs (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id UUID NOT NULL,
+    name VARCHAR(200) NOT NULL,
+    url VARCHAR(500) NOT NULL,
+    api_token VARCHAR(500),
+    enabled BOOLEAN DEFAULT true NOT NULL,
+    description TEXT,
+    last_connected_at TIMESTAMPTZ,
+    created_by UUID,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_mcp_server_configs_tenant ON mcp_server_configs(tenant_id);
+CREATE INDEX IF NOT EXISTS idx_mcp_server_configs_tenant_name ON mcp_server_configs(tenant_id, name);