[PM-5997] Resolve forum QA follow-ups

[jmgasper]

Summary

• PM-6164: limit post deletion to administrators and scoped M2M callers while preserving author edits
• PM-6182: normalize the event-bus configuration to the v6 base, remove the broken /eventBus fallback, and cover the watch-to-email publication path
• PM-6183: project challenge Copilot roles in one batched resource lookup and expose authorIsCopilot for the UI badge
• PM-6185: return top-level posts and nested replies oldest-first with stable ordering

Validation

• pnpm test --runInBand — 135 passed, 3 skipped
• pnpm lint
• pnpm build

Deployment / QA note for PM-6182

The code path now resolves the wrapper base to https://api.topcoder-dev.com/v6, but deployment still needs non-empty SENDGRID_NOTIFICATION_TEMPLATE, M2M client credentials with write:bus_api, and either the canonical Bus v6 value or TOPCODER_API_URL_BASE. The triggering post author is intentionally excluded from watch email recipients, so QA should retest with separate watcher and posting accounts and identify the topic/account/time if it still fails.