Notification delivery: invite emails (SES), @mention notifications and notification preferences

Two notifications are queued through the outbox today, but nothing sends them:

• Invite emails: workspace.invite.email is handled by InviteEmailHandler, which only logs and completes until an InviteMailSender bean exists.
• Mentions: comment.mention is handled by CommentMentionHandler, which only logs and completes until a MentionNotifier bean exists.

Also, Settings → Account → Notifications is a placeholder (PAR-384).

Do

• An InviteMailSender using AWS SES (gitops already has an SES sending setup). Templated sentence-case emails with the /invite/<token> link.
• After delivery, purge the raw token from delivered workspace.invite.email payloads. There's a TODO(PAR-383) about this in InviteEmailHandler.
• A MentionNotifier that sends an email and/or a Discord DM (linked accounts), respecting user preferences.
• A notification preferences backend in the workspaces domain (per user: email, Discord DM, which events), plus the real Settings → Notifications page.
• Tests: handlers with fake senders, idempotency on retry, token purge.