feat(5.2): add User Preferences API with full-stack implementation

Backend:
- Create app/models/user_preference.py with TenantMixin (user_id, key, value JSONB)
- Create app/routes/user_preferences.py with GET/PUT/DELETE endpoints + RBAC
- Add user_preferences:read/write to CORE_PERMISSIONS
- Add user_preferences to legacy role permissions (admin/editor/viewer)
- Register route in app/main.py and app/routes/__init__.py
- Create alembic migration 0028_user_preferences
- Add UserPreference model to conftest.py for test schema
- Fix pre-existing conftest seed (Contact industry field removed in migration 0027)

Frontend:
- Create frontend/src/api/userPreferences.ts with React Query hooks
- Create frontend/src/hooks/useUserPreferences.ts syncing with uiStore
- Add i18n entries for de.json and en.json

Tests:
- 13 tests covering CRUD, tenant isolation, CSRF, unauthenticated access
- All tests passing

app/core/permission_registry.py

@@ -41,6 +41,8 @@ CORE_PERMISSIONS: list[dict[str, str]] = [
     {"key": "attachments:delete", "label": "Attachments: Delete", "category": "core", "module": "attachments"},
     {"key": "workflows:read", "label": "Workflows: Read", "category": "core", "module": "workflows"},
     {"key": "workflows:write", "label": "Workflows: Write", "category": "core", "module": "workflows"},
+    {"key": "user_preferences:read", "label": "User Preferences: Read", "category": "core", "module": "user_preferences"},
+    {"key": "user_preferences:write", "label": "User Preferences: Write", "category": "core", "module": "user_preferences"},
     {"key": "sequences:read", "label": "Sequences: Read", "category": "core", "module": "sequences"},
     {"key": "sequences:write", "label": "Sequences: Write", "category": "core", "module": "sequences"},
     {"key": "addresses:read", "label": "Addresses: Read", "category": "core", "module": "addresses"},