fix: make complete_review idempotent and return 409 on re-completion (issue #1873)

[ionfwsrijan]

Problem

complete_review is not idempotent: calling it twice on the same review recomputes the score, overwrites completed_at/overall_score/status, and increments submission["review_count"] a second time. A client retry or double-click silently inflated the review count and re-stamped the completed review, even though the API always returned 200.

Fix

• Added ReviewAlreadyCompletedError in src/utils/code_review.py.
• complete_review now raises ReviewAlreadyCompletedError when review["completed_at"] is already set, before recomputing scores or touching the submission. review_count is only ever incremented on the transition from a non-completed state.
• The route catches the new exception and returns HTTP 409 Conflict; the completed review and its review_count/metrics remain untouched.

Files changed

• src/utils/code_review.py — new ReviewAlreadyCompletedError; idempotency guard in complete_review.
• src/routes/main_routes.py — catch ReviewAlreadyCompletedError and return 409.
• tests/test_code_review.py — regression test asserting double-completion raises the error, does not inflate review_count, and does not restamp completed_at/overall_score/summary.

Testing

• Isolated test run of tests/test_code_review.py — 20 passed (19 existing + 1 new).
• Route simulation confirmed first complete returns 200 (review_count=1), second complete returns 409 and leaves review_count=1.

Closes #1873

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