feat: MCP Server plugin (Task 5.16) - expose LeoCRM tools to external AI clients

- New plugin app/plugins/builtins/mcp_server/ with 9 MCP tools
- Tools: search_contacts, get_contact, create_contact, list_calendar_entries,
  create_calendar_entry, list_emails, send_email, list_files, upload_file
- Routes: GET /api/v1/mcp/tools, POST /api/v1/mcp/tools/{name}/execute, GET /api/v1/mcp/config
- API-token auth via session + RBAC (mcp:read, mcp:write)
- Frontend: mcp.ts API client with React Query hooks
- Frontend: SettingsMcp.tsx settings page with tool listing and execution
- i18n: de.json and en.json updated with MCP entries
- Tests: 7 tests covering tool listing, config, execution, auth, schema validation

app/plugins/builtins/mcp_server/schemas.py

@@ -0,0 +1,54 @@
+"""Pydantic schemas for MCP Server plugin."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from pydantic import BaseModel, Field
+
+
+class McpToolParameter(BaseModel):
+    """Single parameter in an MCP tool definition."""
+    name: str
+    type: str
+    description: str
+    required: bool = False
+    default: Any | None = None
+
+
+class McpToolDefinition(BaseModel):
+    """MCP tool definition returned by GET /api/v1/mcp/tools."""
+    name: str
+    description: str
+    category: str
+    parameters: list[McpToolParameter]
+    required_permission: str | None = None
+
+
+class McpToolExecuteRequest(BaseModel):
+    """Request body for POST /api/v1/mcp/tools/{tool_name}/execute."""
+    arguments: dict[str, Any] = Field(default_factory=dict)
+
+
+class McpToolExecuteResponse(BaseModel):
+    """Response from executing an MCP tool."""
+    tool: str
+    success: bool
+    result: Any | None = None
+    error: str | None = None
+
+
+class McpServerConfig(BaseModel):
+    """MCP server configuration for external clients."""
+    server_name: str = "LeoCRM"
+    server_version: str = "1.0.0"
+    protocol_version: str = "2024-11-05"
+    base_url: str = "/api/v1/mcp"
+    auth_method: str = "api-token"
+    available_tools: list[str] = Field(default_factory=list)
+
+
+class McpToolListResponse(BaseModel):
+    """Response listing all MCP tools."""
+    tools: list[McpToolDefinition]
+    count: int