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/main.py

@@ -11,6 +11,7 @@ from app.db.session import engine
 from app.invoices.router import router as invoices_router
 from app.orders.router import router as orders_router
 from app.sync.router import router as sync_router
+from app.users.router import router as users_router
 from app.vehicles.router import router as vehicles_router
 
 # Import models so Base.metadata knows about them
@@ -37,6 +38,7 @@ app.include_router(sync_router, prefix="/api/sync")
 app.include_router(orders_router, prefix="/api/orders")
 app.include_router(vehicles_router, prefix="/api/vehicles")
 app.include_router(invoices_router, prefix="/api/invoices")
+app.include_router(users_router, prefix="/api/users")
 app.include_router(portal_router, prefix="/api")