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/main.py

@@ -40,6 +40,7 @@ from app.routes import (
     roles,
     tenants,
     users,
+    user_preferences,
     workflows,
     currencies,
     taxes,
@@ -239,6 +240,7 @@ def create_app() -> FastAPI:
     app.include_router(plugins.router)
     app.include_router(ai_copilot.router)
     app.include_router(workflows.router)
+    app.include_router(user_preferences.router)
     app.include_router(currencies.router)
     app.include_router(taxes.router)
     app.include_router(sequences.router)