Refactor and improve code consistency across multiple files

- Enhanced SQL queries in `session.go` and `share.go` for clarity and consistency.
- Updated comments for better understanding and maintenance.
- Ensured consistent error handling and return statements across various methods.

backend/internal/auth/jwt.go

@@ -60,4 +60,4 @@ func ValidateJWT(tokenString, secret string) (*UserClaims, error) {
 	}
 
 	return nil, errors.New("invalid token claims")
-}
+}