feat: add username-password login flow via authentik token endpoint

backend/app/schemas/login.py

@@ -0,0 +1,13 @@
+from pydantic import BaseModel
+
+
+class LoginRequest(BaseModel):
+    username: str
+    password: str
+
+
+class LoginResponse(BaseModel):
+    access_token: str
+    token_type: str = "Bearer"
+    expires_in: int | None = None
+    scope: str | None = None