feat: newsletter/channel support

Add WhatsApp Newsletter (Channels) support:

- `get_newsletter_metadata(jid)` — get channel info
- `subscribe_newsletter(jid)` / `unsubscribe_newsletter(jid)` — follow/unfollow
- `post_to_newsletter(jid, content)` — post to a channel (admin)
- Inbound: surface newsletter messages

Requires adjusting `should_ignore_jid` to selectively allow @newsletter JIDs when opted in.