fix(api): don't double-run webhook side effects on a concurrent duplicate

[cavidelizade]

Summary

Webhook deduplication was a check-then-act: ExistsByDeliveryID then Create. Two concurrent deliveries of the same GitHub event both pass the exists check; one Create wins and the other fails the unique delivery_id constraint. That error was only logged, so the losing racer fell through to dispatch and ran the side effects a second time, giving duplicate bot comments and duplicate or oscillating state changes.

Linked issues

Closes #332

Type of change

• Bug fix (fix:)

Surface

• API (apps/api/)

What changed

Treat a duplicate-key error from Create (gorm.ErrDuplicatedKey, already used elsewhere with TranslateError on) as "already processed" and return before dispatching. Sequential redeliveries were already short-circuited by the exists check; this closes the concurrent window.

Test plan

• go build, go vet, go test ./... green

AI assistance

• AI tools were used, tool(s): Claude Code (Opus 4.8), commits carry a Co-Authored-By: trailer

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Review details

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Configuration used: Organization UI

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Run ID: 170e4d08-1bf8-4887-845e-56d49b66abe3

📥 Commits

Reviewing files that changed from the base of the PR and between 61bc903 and a000aa4.

📒 Files selected for processing (1)

• apps/api/internal/service/github_events.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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