Delete broadcasts (with archive for sent ones) #147

Broadcasts can't be deleted from the dashboard or MCP. DELETE /sequences/:sequenceId exists in the REST API, but nothing calls deleteSequence in apps/web/lib/api.ts, and there is no MCP tool for it.

Risk in the current API: deleteSequence (apps/api/src/sequences/queries.ts) has no status guard. Because email_deliveries and ongoing_sequences use ON DELETE CASCADE on sequences.id, deleting a broadcast that is sending or already sent removes its delivery history and report.

Scope

• API: allow hard delete only for drafts and scheduled broadcasts that haven't started. Once sending has started or finished, return 409 with a code (for example broadcast_sent). Sent broadcasts get archive instead (hidden from the list by default, report and web-archive page kept), as Mailchimp does. Update the OpenAPI docs.
• MCP: add a delete_broadcast tool (and archive_broadcast if archive ships here).
• Dashboard: a Delete action in the broadcasts list row menu and on the detail page, with a shadcn AlertDialog confirmation. Disable it, with a reason, when delete isn't allowed.
• Observability: sequence_deleted already fires. Add an archive event if archive ships.
• Web archive: a deleted or archived broadcast is removed from /w/:handle, the feed and the sitemap.