feat: add authentik admin user sync endpoint

backend/app/schemas/authentik_admin.py

@@ -0,0 +1,13 @@
+from pydantic import BaseModel
+
+
+class AuthentikEnsureUserRequest(BaseModel):
+    sub: str
+    email: str
+    display_name: str | None = None
+    is_active: bool = True
+
+
+class AuthentikEnsureUserResponse(BaseModel):
+    authentik_user_id: int
+    action: str