fix(profile): validate username uniqueness before profile updates

[sujitha-kotyada]

📋 PR Checklist

🔗 Related Issue

Closes #673

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📝 What does this PR do?

This PR fixes a bug in the PUT /profile endpoint where username updates bypassed application-level uniqueness validation.

Previously, if a user attempted to change their username to one already used by another account, the database unique constraint would trigger an unhandled IntegrityError, resulting in a 500 Internal Server Error.

Changes made:

• Added username uniqueness validation before updating the user record.
• Excluded the current user from duplicate username checks.
• Added IntegrityError handling with transaction rollback to safely handle race conditions.
• Returned a user-friendly validation error instead of exposing a database exception.
• Added regression tests covering duplicate, unchanged, and unique username update scenarios.

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Tested the affected API endpoints manually
• Added / updated tests

Validation Performed

• Added regression tests for:

  • Duplicate username update attempt
  • Keeping the existing username
  • Updating to a new unique username

• Executed:

  backend/.venv/Scripts/pytest backend/tests/test_profile.py

• Executed full backend test suite:

  backend/.venv/Scripts/pytest

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📸 Screenshots (if UI change)

N/A (Backend-only change)

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⚠️ Anything to flag for reviewers?

• No schema or database migrations were required.
• The change is isolated to the profile update endpoint.
• Added defensive IntegrityError handling to protect against concurrent update race conditions.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed