feat: add incremental analysis endpoint

[bhavyajain0810]

Description

Adds backend support for incremental analysis so the app can analyze only changed files or changed code hunks instead of re-analyzing the full project every time.

Implemented:

• Added a new POST /analyze/incremental/ endpoint
• Added request/response schemas for incremental analysis
• Added diff-based changed line detection using difflib.SequenceMatcher
• Added support for added, modified, renamed, deleted, and unchanged files
• Skips unchanged files and deleted files where no new code needs analysis
• Handles renamed files using previous_path
• Falls back to full-file analysis when a large portion of the file has changed
• Returns metadata such as changed line ranges, changed line count, analyzed/skipped file count, and analysis time
• Added backend tests for modified, added, deleted, and renamed file cases

Related Issue

Fixes #670

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

Not applicable. This PR adds a backend API endpoint and tests only.

Test evidence

python -m ruff format backend/app/schemas.py backend/app/services/incremental_analysis.py backend/app/routers/analyze.py backend/tests/test_endpoints.py
# 4 files reformatted

python -m ruff check backend/app --select E,F,W --ignore E501 --fix
# All checks passed!

python -m ruff check backend/app --select E,F,W --ignore E501
# All checks passed!

cd backend
pytest -v
# 376 passed, 2 warnings in 14.19s

Ruff check evidence:

Pytest evidence: