#3932 Disable the inbox notification email ("You have received a notification in Swamp Club")

Description

Every new inbox notification (targeted messages, admin broadcasts, Lab mentions, issue shipped/closed) enqueues a blanket Resend email, "You have received a notification in Swamp Club", with an "Open your inbox" CTA. This shipped in #1296 and we want it turned off. The in-app inbox stays as it is; only the email goes.

Where it lives

• lib/app/send-inbox-email.ts: subject line and enqueueInboxEmails
• Enqueue call sites: lib/app/send-system-message.ts, lib/app/admin-message-log.ts, lib/app/process-mentions.ts, lib/app/notify-issue-resolved.ts
• Queue + drain: lib/infrastructure/mongo-inbox-email-queue.ts (inbox_email_queue), lib/infrastructure/inbox-email-queue-watcher.ts

Expected

A new inbox notification produces no email. Nothing is enqueued, so no backlog builds up that would send if the feature came back. Rows already in inbox_email_queue must not send either. The drain drops rows older than 3 days, but anything newer than that is still waiting to go out.

Steps to reproduce

1. Send an operative a system message, @-mention them in a Lab issue, or ship an issue they authored.
2. The operative gets the "You have received a notification in Swamp Club" email.

Environment

Production swamp-club.com, with RESEND_API_KEY and the unsubscribe secret set (that is what gates the drain).