fix(security): add authentication to history routes

[anshul23102]

Summary

This PR fixes the critical authentication gap in history routes by requiring authentication on all endpoints and filtering history records by current user. Users can now only access their own analysis history.

Problem

The history routes in backend/app/routers/history.py lacked authentication, exposing all users' code analysis history to any client. Unauthenticated requests could retrieve, search, and delete history entries belonging to other users.

Solution

1. Added authentication requirement using Depends(get_current_user) to all history endpoints
2. User-scoped filtering - history records filtered by current_user.id
3. User tracking - save_entry() now stores user_id for proper data isolation
4. Delete protection - delete operations now verify user ownership via user_id

Changes

• backend/app/routers/history.py:
  • Import Depends and get_current_user from security
  • Add current_user parameter to all route handlers
  • Filter all database queries by user_id
  • Include user_id when saving new entries

Security Impact

• Prevents unauthorized access to other users' analysis history
• Ensures complete user data isolation
• Protects sensitive code snippets from exposure

Testing

• Authenticated users can access only their own history
• Unauthenticated requests return 401 Unauthorized
• Users cannot delete or search history belonging to other users
• History entries are properly associated with creating user

Related Issue

Closes #708

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This contribution is part of GSSoC 2026. Please consider adding the gssoc-approved label when reviewed.