fix(backend): replayed named creates return existing row (#1320)

[LucasSantana-Dev]

Closes #1320.

Custom-command and embed-template POSTs hit @@unique([guildId, name]) on replay (retry-after-flake, double-click) and surfaced a P2002-derived error even though the first request succeeded.

• Catch P2002 in the two create routes, fetch the existing row, return it with 200. Idempotent success per the house pattern in decisions/2026-06-12-idempotency-posture.md (PR docs: add idempotency posture decision #1325).
• Documented choice on genuine conflicts (same name, different payload): same path — the existing record is returned and the client edits via PATCH. No 409, no payload diffing.
• The created server log is skipped on replay since nothing was created.
• If the existing row vanished between P2002 and the fetch (delete race), the original error propagates — no silent 200 with an empty body.
• Specs per route: replay returns existing + skips log; vanished-row race rethrows; embeds lookup uses the stored lowercased name.

Backend suite: 1000/1000.

---

Summary by cubic

Make named create routes idempotent. Replayed POSTs that hit the unique (guildId, name) return the existing record with 200 instead of a P2002 error for custom commands and embed templates (#1320).

• Bug Fixes
  • Catch P2002 in both create routes; fetch and return the existing row (200).
  • Skip "created" server log on replays; if payload differs, edit via PATCH (no 409).
  • For embeds, look up by the stored lowercased name.
  • If the row disappears between P2002 and fetch, rethrow (no silent 200).
  • Add isUniqueViolation helper and integration tests for replay, delete race, and lowercase lookup.

^{Written for commit ec62d97. Summary will update on new commits.}

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Project	Deployment	Actions	Updated (UTC)
lucky	Ready	Preview, Comment	Jun 12, 2026 12:19pm

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 77c8a65f-7dcc-4bcb-a9fa-e2e4fc65926b

📥 Commits

Reviewing files that changed from the base of the PR and between fc1b094 and ec62d97.

📒 Files selected for processing (5)

• packages/backend/src/routes/management.ts
• packages/backend/src/routes/managementEmbeds.ts
• packages/backend/src/utils/prismaErrors.ts
• packages/backend/tests/integration/routes/management.test.ts
• packages/backend/tests/integration/routes/managementEmbeds.test.ts

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• Create PR with unit tests
• Commit unit tests in branch fix/1320-idempotent-named-creates

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
87.9% Coverage on New Code
0.0% Duplication on New Code

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