feat(auth): add admin bootstrap and username login

Initialize admin bootstrap settings during startup, persist username support in auth flows, and align frontend auth requests with local API behavior.

backend/app/schemas/auth.py

@@ -2,6 +2,7 @@ from pydantic import BaseModel, EmailStr
 
 
 class UserCreate(BaseModel):
+    username: str
     email: EmailStr
     password: str
     full_name: str | None = None
@@ -9,6 +10,7 @@ class UserCreate(BaseModel):
 
 class UserOut(BaseModel):
     id: str
+    username: str
     email: str
     full_name: str | None
     is_active: bool