fix(sentry): null-coalesce PostgREST details/hint to prevent TypeError

[sw-factory-automations]

Summary

PostgREST 500 responses return details: null and hint: null. The isPostgrestError duck-type check uses the in operator, which returns true when the key exists regardless of value type. Calling null.startsWith() or null.includes() in isTransientNetworkError and isSupabaseAuthLockError crashes the retry logic, turning a retryable Supabase 500 into an unhandled rejection that crashes the page.

Sentry issue: https://ona-2j.sentry.io/issues/MEMO-2E — 2 events, 1 user, unhandled.

Closes #944

Root Cause

1. Supabase returns HTTP 500 for a pages query
2. retryOnNetworkError() calls isTransientNetworkError(result.error)
3. isPostgrestError() returns true (all 4 keys exist on the error object)
4. error.details is null (PostgREST 500 response)
5. null.startsWith() → TypeError: Cannot read properties of null (reading 'startsWith')

Changes

• src/lib/sentry.ts: Update isPostgrestError type guard to reflect that details and hint can be null. Add ?? "" null coalescing in isTransientNetworkError, isSupabaseAuthLockError, and captureSupabaseError.
• src/lib/sentry.unit.test.ts: Add regression tests for PostgREST errors with details: null and hint: null across all three affected functions.
• .agents/conventions.md: Document the null-safety convention for PostgREST fields.

Testing

• pnpm lint — 0 errors
• pnpm typecheck — passes
• pnpm test — 1812 tests pass (132 files)

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Project	Deployment	Actions	Updated (UTC)
memo	Ready	Preview, Comment	May 7, 2026 7:22pm

[sw-factory-automations]

✅ UI verification skipped — no UI files changed. This PR only modifies src/lib/sentry.ts (utility logic), its unit tests, and .agents/conventions.md (documentation).

[sw-factory-automations]

✅ Post-merge verification passed.

E2E tests against live site (https://memo.software-factory.dev):

• e2e/auth.spec.ts — 8/8 passed (sign-in, sign-up, sign-out, redirects)
• e2e/page-crud.spec.ts — 5/5 passed (create, navigate, rename, delete pages)
• e2e/database-crud.spec.ts — 11/11 passed (create DB, add columns, edit cells, delete rows/columns, undo)

Ad-hoc smoke tests:

• ✅ Landing page — loads, has title
• ✅ Sign-in page — renders with email input
• ✅ Health endpoint (/api/health) — returns OK
• ✅ Authenticated flow — login succeeds, workspace page loads
• ✅ No console errors (unauthenticated or authenticated)

Skipped:

• /dashboard (route does not exist — 404)
• Editor navigation (no page buttons with timestamps found in test workspace)