Add admin account with user management endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/config.py

@@ -6,6 +6,8 @@ class Settings(BaseSettings):
     secret_key: str
     access_token_expire_minutes: int = 480
     algorithm: str = "HS256"
+    admin_username: str
+    admin_password: str
 
     class Config:
         env_file = ".env"