feat: add hotel_admin (Системный администратор) and technician roles + protect hotel owner from deletion

- Migration 035: extend role constraint to include hotel_admin and technician; migrate existing manager users to hotel_admin
- auth.ts: registration now assigns hotel_admin (not manager) to hotel owner
- seed.ts: demo user manager@grand-palace.ru seeded as hotel_admin
- types.ts: expand JwtPayload role union with all roles
- users.ts: hotel_admin included in access checks; role creation/edit/delete rules enforced; hotel_admin users are undeletable and uneditable (non-super_admin); role cannot be set to hotel_admin via PATCH
- rooms.ts / channels.ts: hotel_admin added to write-access checks
- UsersPage.tsx: hotel_admin and technician added to StaffRole, ROLE_META, DEFAULT_POSITIONS, mapRole, backendRoleMap, INITIAL_ROLE_PERMISSIONS; delete button hidden for hotel_admin; role selector locked for hotel_admin users; hotel_admin excluded from new-user role selector

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/035_hotel_admin_technician.sql

@@ -0,0 +1,7 @@
+-- Extend role constraint to include hotel_admin and technician
+ALTER TABLE users DROP CONSTRAINT IF EXISTS users_role_check;
+ALTER TABLE users ADD CONSTRAINT users_role_check
+  CHECK (role IN ('hotel_admin', 'manager', 'housekeeper', 'super_admin', 'receptionist', 'accountant', 'security', 'technician'));
+
+-- Migrate existing manager users (hotel owners) to hotel_admin
+UPDATE users SET role = 'hotel_admin' WHERE role = 'manager';