Prevent users from submitting duplicate code (Plagiarism Detection)

Description:
Currently, users can submit the same code multiple times without restriction. This can lead to spam, unfair usage, and potential plagiarism.

We need to implement a system that detects and prevents duplicate code submissions by the same user.

Proposed Solution:

Generate a hash (e.g., SHA-256) of submitted code
Store the hash in the database along with user_id
Add a unique constraint (user_id + code_hash)
Reject submission if the same hash already exists

Optional Enhancements:

Normalize code before hashing (remove whitespace/comments)
Add time-based restriction (e.g., prevent duplicates within 30 days)
Extend to cross-user plagiarism detection (future scope)

Acceptance Criteria:

User cannot submit identical code more than once
System efficiently checks duplicates even at scale
Proper error message is returned on duplicate submission

Tech Considerations:

Use indexed database fields for fast lookup
Ensure hashing is consistent and collision-safe
Backend validation only (not frontend)