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/models.py

@@ -0,0 +1,35 @@
+"""MCP Client plugin models — external MCP server configurations."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, Index, String, Text
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class McpServerConfig(Base, TenantMixin):
+    """Configuration for an external MCP server — tenant-scoped."""
+
+    __tablename__ = "mcp_server_configs"
+    __table_args__ = (
+        Index("ix_mcp_server_configs_tenant", "tenant_id"),
+        Index("ix_mcp_server_configs_tenant_name", "tenant_id", "name"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    url: Mapped[str] = mapped_column(String(500), nullable=False)
+    api_token: Mapped[str | None] = mapped_column(String(500), nullable=True)
+    enabled: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    last_connected_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    created_by: Mapped[uuid.UUID | None] = mapped_column(PGUUID(as_uuid=True), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=datetime.utcnow, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)