feat: Add Matrix user provisioning on registration

- matrix_provisioning.py: Create Matrix users via Synapse admin API
- Auto-create Matrix account when user registers in DAARION
- Return matrix_user_id in registration response

services/auth-service/models.py

@@ -47,6 +47,7 @@ class RegisterResponse(BaseModel):
     email: str
     display_name: Optional[str] = None
     roles: List[str] = ["user"]
+    matrix_user_id: Optional[str] = None
 
 
 class TokenResponse(BaseModel):