No length validation between questions/user_answers/correct_answers

[DeFiVC]

What

The feedback request model has no cross-validation ensuring len(questions) == len(user_answers) == len(correct_answers):

questions: list[QuizQuestion] = Field(...)
user_answers: list[int] = Field(...)
correct_answers: list[int] = Field(...)

In feedback_engine.py line 44-46, zip(user_answers, correct_answers) silently truncates to the shorter list, causing answers to be silently ignored and incorrect feedback/score computation.

Why

Mismatched list lengths cause silent data loss and incorrect feedback.

Scope

• Add a Pydantic validator to ensure all three lists have the same length

Acceptance Criteria

• Requests with mismatched list lengths are rejected
• Clear error message is provided

Technical Context

• File: src/routes/feedback.py, lines 20-22
• Use @model_validator or @validator to cross-validate