feat(UI-Overhaul-Phase6): Tags Umstrukturierung

Migration 0138: Add parent_id, applicable_to, icon columns to tags table

Backend:
- Tag model: add parent_id (self-FK), applicable_to (JSONB), icon (VARCHAR)
- TagCreate/TagUpdate/TagResponse schemas: add new fields
- Tags routes: create_tag, update_tag, list_tags return new fields

Frontend:
- api/tags.ts: Tag interface, CreateTagPayload, UpdateTagPayload updated with new fields
- Tags route moved from /tags to /settings/tags (under Settings)
- Tags.tsx: TagFormModal updated with parent tag selector, icon picker, applicable_to multi-select
- TagsPage passes tags list to TagFormModal for parent selection

tsc clean, backend import OK

frontend/src/routes/index.tsx

@@ -259,7 +259,7 @@ const router = createBrowserRouter([
       { path: '/workflows', element: <PermissionRoute permission="workflows:read">{withSuspense(<WorkflowsPage />)}</PermissionRoute> },
       { path: '/contacts/dedup', element: <PermissionRoute permission="contacts:read">{withSuspense(<DedupMergePage />)}</PermissionRoute> },
       { path: '/import-export', element: <PermissionRoute permission="contacts:read">{withSuspense(<ImportExportPage />)}</PermissionRoute> },
-      { path: '/tags', element: <PermissionRoute permission="tags:read">{withSuspense(<TagsPage />)}</PermissionRoute> },
+      { path: 'tags', element: <PermissionRoute permission="tags:read">{withSuspense(<TagsPage />)}</PermissionRoute> },
       { path: '/api-docs', element: <PermissionRoute permission="settings:read">{withSuspense(<ApiDocsPage />)}</PermissionRoute> },
       { path: '/activity', element: <PermissionRoute permission="activity:read">{withSuspense(<ActivityTimelinePage />)}</PermissionRoute> },
       { path: '/wiki', element: withSuspense(<WikiPage />) },