WIP: soundManager lazy import, requestId concurrency tests, pauseGuard tests, webhooks.md cross-check

[Themancalledpg]

Draft placeholder — claiming this batch of 4 assigned issues, real implementation to follow. No code changes yet.

• [Frontend] frontend/src/app/utils/soundManager.ts is imported eagerly even when sound is disabled #1523 — frontend/src/app/utils/soundManager.ts is imported eagerly even when sound is disabled
• [Backend] backend/src/middleware/requestId.ts uniqueness under concurrency is untested #1522 — backend/src/middleware/requestId.ts uniqueness under concurrency is untested
• [Testing] backend/src/middleware/pauseGuard.ts has no dedicated unit test #1521 — backend/src/middleware/pauseGuard.ts has no dedicated unit test
• [Docs] docs/webhooks.md event type list is not cross checked against webhookService.ts #1520 — docs/webhooks.md event type list is not cross checked against webhookService.ts

closes #1520
closes #1521
closes #1522
closes #1523

[Themancalledpg]

Closing as a duplicate — #1569 already covers this exact issue set (#1520, #1521, #1522, #1523) with real implementation work. This draft placeholder is redundant.