Harden webhook migration concurrency

[haasonsaas]

Summary:

• Catch concurrent constraint creation races in the webhook delivery reconciliation migration.
• Guard concurrent index creation races observed in disposable Postgres.
• Extend migration packaging assertions for concurrency handlers.

Internal source PR: evalops/maestro-internal#1515

Test plan:

• npm run test -- test/web-health.test.ts test/db/migration-files.test.ts test/db/migrate-legacy-reconciliation.test.ts
• bunx biome check test/db/migration-files.test.ts
• git diff --check
• Disposable Postgres 16: ran 0008 migration twice concurrently against a drifted legacy webhook_deliveries schema and verified helper, PK, FK, indexes, status normalization, and invalid payload wrapping.

[cursor]

PR Summary

Medium Risk
Touches a production database migration by adding exception-handling around constraint/index creation; behavior should be unchanged, but migration execution paths can vary across Postgres versions and drifted schemas.

Overview
Hardens the 0008_reconcile_legacy_webhook_deliveries migration against concurrent runs by wrapping primary key / foreign key constraint creation in BEGIN … EXCEPTION blocks to ignore duplicate/invalid-definition races.

Similarly guards index creation on webhook_deliveries to tolerate concurrent CREATE INDEX IF NOT EXISTS races (duplicate_table/unique_violation). Tests are updated to assert these new concurrency-handling clauses are present in the packaged migration SQL.

^{Reviewed by Cursor Bugbot for commit 0ffd0f7. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

This PR changes mirrored Maestro source files in the public repo, but it does not link the matching private source-of-truth PR.

Add one of these to the PR body, then re-run the check:

• https://github.com/evalops/maestro-internal/pull/<number>
• evalops/maestro-internal#<number>
• maestro-internal#<number>

Mirrored files touched:

• src/db/migrations/0008_reconcile_legacy_webhook_deliveries.sql
• test/db/migration-files.test.ts