feat(backend): invite system + user management

- InviteToken model with unique token for each invite
- POST /users/invite - create invite by email with role (admin/mecanic)
- POST /auth/accept-invite - accept invite, set password, return JWT
- GET /users - list all users in tenant
- DELETE /users/{id} - deactivate user (cannot deactivate owner)
- Alembic migration for invites table
- 25 passing tests (auth + sync + orders + pdf + portal + invoices + users)

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

backend/app/db/models/__init__.py

@@ -15,6 +15,7 @@ from app.db.models.catalog import (
 from app.db.models.invoice import Invoice
 from app.db.models.appointment import Appointment
 from app.db.models.mecanic import Mecanic
+from app.db.models.invite import InviteToken
 
 __all__ = [
     "Tenant",
@@ -32,4 +33,5 @@ __all__ = [
     "Invoice",
     "Appointment",
     "Mecanic",
+    "InviteToken",
 ]