fix(api): close inactive threads automatically

[mikemikimike]

Summary

Fixes #1960.

• Add a scheduled cleanup that closes open threads after each source's thread_inactivity_hours threshold.
• Run cleanup once at application startup and then every minute, reusing the existing closeThread lifecycle.
• Share the inactivity predicate with incoming-message thread resolution and cover exact-threshold, per-source settings, scheduler, shutdown, and failure behavior.

Validation

Validated from a clean Linux checkout of this PR commit using Node.js 24.17.0 and pnpm 11.8.0:

• pnpm --filter @hexabot-ai/api run test: 178 suites passed, 1 skipped; 1,234 tests passed, 3 skipped.
• pnpm --filter @hexabot-ai/api run lint: passed.
• pnpm --filter @hexabot-ai/api run typecheck: passed.
• pnpm --filter '@hexabot-ai/api...' run build: passed for the API and its workspace dependency chain.
• Focused thread service and scheduler regression tests: 21 passed.
• git diff --check: passed.