feat: JWT auth, user management, audit logging system

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

backend/app/schemas/auth.py

@@ -0,0 +1,23 @@
+from pydantic import BaseModel
+
+
+class LoginRequest(BaseModel):
+    username: str
+    password: str
+
+
+class RegisterRequest(BaseModel):
+    username: str
+    email: str  # 先用 str，后续升级 EmailStr
+    password: str
+
+
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+    expires_in: int = 0
+
+
+class ChangePasswordRequest(BaseModel):
+    old_password: str
+    new_password: str