feat(poster): bump nntppool to v4.11.1 and use PostHeaders.Date field

[javi11]

Summary

• Bumps nntppool from v4.10.1 to v4.11.1
• Moves the article date out of Extra["Date"] (pre-formatted string) and into the new PostHeaders.Date time.Time field introduced in v4.11.1
• The library now handles RFC-compliant date formatting internally, falling back to time.Now().UTC() if the field is zero

What changed

internal/poster/poster.go — postArticleWithBody:

// Before
headers := nntppool.PostHeaders{ ... }
headers.Extra["Date"] = []string{art.Date.UTC().Format(time.RFC1123Z)}

// After
headers := nntppool.PostHeaders{
    ...
    Date:  art.Date.UTC(),
    ...
}

Reviewer notes

Behaviour is identical — the article date is still applied to every outgoing article. The only difference is that formatting is now delegated to the library, which is the idiomatic way per the v4.11.1 PostHeaders documentation.

Test plan

• go build ./internal/poster/... — clean
• go test ./internal/poster/... — all tests pass