fix: Update Pydantic models to use ConfigDict instead of deprecated class-based config

Replace deprecated class Config with model_config = ConfigDict() to fix Pydantic v2 deprecation warnings

backend/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI, HTTPException, Header, Depends
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import RedirectResponse
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ConfigDict
 from typing import List, Optional
 from datetime import datetime
 from sqlalchemy.orm import Session
@@ -46,31 +46,31 @@ app.add_middleware(
 
 # Pydantic Models for API
 class UserResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    
     id: str
     username: str
     email: str
     created_at: datetime
 
-    class Config:
-        from_attributes = True
-
 class UserRegister(BaseModel):
     username: str
     email: str
     password: str
 
 class UserLogin(BaseModel):
+    model_config = ConfigDict(populate_by_name=True)
+    
     username_or_email: str = Field(..., alias='usernameOrEmail')
     password: str
-    
-    class Config:
-        populate_by_name = True
 
 class AuthResponse(BaseModel):
     user: UserResponse
     token: str
 
 class TaskListResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    
     id: str
     user_id: str
     name: str
@@ -78,9 +78,6 @@ class TaskListResponse(BaseModel):
     color: str = "#667eea"
     created_at: datetime
 
-    class Config:
-        from_attributes = True
-
 class TaskListCreate(BaseModel):
     name: str
     icon: Optional[str] = "📝"
@@ -92,6 +89,8 @@ class TaskListUpdate(BaseModel):
     color: Optional[str] = None
 
 class TaskResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    
     id: str
     list_id: str
     user_id: str
@@ -102,9 +101,6 @@ class TaskResponse(BaseModel):
     created_at: datetime
     updated_at: datetime
 
-    class Config:
-        from_attributes = True
-
 class TaskCreate(BaseModel):
     title: str
     list_id: str