fix(logging): use object cause in TRPCErrors so event name is logged correctly

[danadajian]

Summary

• TRPCError's getCauseFromUnknown helper wraps string cause values in new Error(String(cause)), causing JSON.stringify to serialize them as {} (since Error properties are non-enumerable)
• Changed all cause: 'EVENT_NAME' strings to cause: { event: 'EVENT_NAME' } objects — tRPC then uses Object.assign(new UnknownCauseError(), cause), making event an enumerable property
• Updated server.ts to spread cause (...cause) so the event key surfaces as a top-level field in structured log output alongside path, code, and message

Changes

• app/backend/src/getGroupedKeys.ts — COMMIT_HASH_NOT_FOUND, NO_NEW_OR_DIFF_IMAGES
• app/backend/src/fetchCurrentPage.ts — PAGE_NOT_FOUND, INVALID_FILE_NAME
• app/backend/src/updateCommitStatus.ts — COMMIT_STATUS_UPDATE_FAILED
• app/backend/src/getOctokit.ts — MISSING_SECRETS_FILE, SECRETS_PARSE_ERROR, MISSING_GITHUB_TOKEN
• app/backend/src/acceptVisualChanges.ts — VISUAL_CHANGE_ACCEPTANCE_BLOCKED, MISSING_IDENTIFIER
• app/server.ts — spread cause into log payload instead of passing as nested object

Test plan

• Trigger a COMMIT_HASH_NOT_FOUND error (e.g. load the app with a non-existent commit hash) and verify server logs show "event":"COMMIT_HASH_NOT_FOUND" as a top-level field rather than "cause":{}
• Confirm existing backend unit tests still pass: bunx nx test backend

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[eg-oss-ci]

🎉 This PR is included in version 1.41.4 🎉

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