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/users/schemas.py

@@ -0,0 +1,11 @@
+from pydantic import BaseModel, EmailStr
+
+
+class InviteRequest(BaseModel):
+    email: EmailStr
+    rol: str = "mecanic"
+
+
+class AcceptInviteRequest(BaseModel):
+    token: str
+    password: str