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/migrations/0001_initial.sql

@@ -0,0 +1,4 @@
+-- MCP Server plugin: no persistent tables needed (stateless tool gateway).
+-- This migration exists to satisfy the plugin migration framework.
+-- MCP tokens are validated via session auth or external API tokens.
+SELECT 1;