Add joined private archived thread listing

[samuelscheit]

Summary

• add GET /channels/:channel_id/users/@me/threads/archived/private
• filter archived private child threads to only those joined by the requesting guild member
• return Discord-compatible thread member objects for the current user and has_more pagination
• cover query construction, limit/before parsing, pagination, permissions, and member serialization with node:test

Root cause

The server had thread creation/member APIs but no route for listing archived private threads joined by the current user, so clients could not page through their joined private archived threads for a parent channel.

Closes #125

Validation

• npm run node:tests
• npm run build:src -- --pretty false
• node -r dotenv/config -r module-alias/register --enable-source-maps --test dist/api/util/utility/JoinedPrivateArchivedThreads.test.js
• npx eslint src/api/routes/channels/#channel_id/users/@me/threads/archived/private.ts src/api/util/utility/JoinedPrivateArchivedThreads.ts src/api/util/utility/JoinedPrivateArchivedThreads.test.ts
• npx prettier --check src/api/routes/channels/#channel_id/users/@me/threads/archived/private.ts src/api/util/utility/JoinedPrivateArchivedThreads.ts src/api/util/utility/JoinedPrivateArchivedThreads.test.ts
• git diff --check

[samuelscheit]

Code review completed after the latest update. I found no blocking issues; the PR is approved from my review.