Refactor token creation logic and fix datetime usage

Adjusted `datetime.utcnow` to `datetime.now` for consistency and refactored token creation functions for cleaner structure. Removed duplicated `create_access_token` and `create_refresh_token` definitions by consolidating them into a single location.

backend/app/auth/security.py

@@ -50,7 +50,6 @@ def create_tokens(user_id: str) -> TokenResponse:
         user_id=user_id
     )
 
-
 def create_token(
         data: dict,
         expires_delta: Optional[timedelta] = None,
@@ -60,9 +59,9 @@ def create_token(
     to_encode = data.copy()
 
     if expires_delta:
-        expire = datetime.utcnow() + expires_delta
+        expire = datetime.now() + expires_delta
     else:
-        expire = datetime.utcnow() + (
+        expire = datetime.now() + (
             timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES) if token_type == "access"
             else timedelta(days=REFRESH_TOKEN_EXPIRE_DAYS)
         )
@@ -70,12 +69,20 @@ def create_token(
     to_encode.update({
         "exp": expire,
         "type": token_type,
-        "iat": datetime.utcnow(),
+        "iat": datetime.now(),
         "jti": str(uuid4())
     })
 
     return jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
 
+def create_access_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
+    """Create a new access token."""
+    return create_token(data, expires_delta, "access")
+
+
+def create_refresh_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
+    """Create a new refresh token."""
+    return create_token(data, expires_delta, "refresh")
 
 def decode_token(token: str, required_type: str = "access") -> TokenPayload:
     """
@@ -130,15 +137,3 @@ def decode_token(token: str, required_type: str = "access") -> TokenPayload:
     except JOSEError as e:  # All other JOSE-related errors
         raise JWTError("Invalid token.") from e
 
-
-
-
-
-def create_access_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
-    """Create a new access token."""
-    return create_token(data, expires_delta, "access")
-
-
-def create_refresh_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
-    """Create a new refresh token."""
-    return create_token(data, expires_delta, "refresh")