Show Slack display name in profile alerts

Profile › Alerts › Channels shows each channel as one line with only the username it delivers to (Task 9168). Telegram shows the linked @name and Email the address, but Slack can only show the raw member id (e.g. U016GGPMR7E): `app/slack_link.py` stores just the sender's id when the code is redeemed. Ask: - When a Slack link code is redeemed, look up the sender's display name (Slack `users.info`) and store it beside the member id. - Show that name on the Slack channel line (Profile › Alerts and the admin's Users › Notifications tab), falling back to the id when no name is stored. Out of scope: Telegram and Email lines; the channel row layout itself; the org-level Slack workspace card. Done means: - Linking Slack on Profile › Alerts shows `@<display name>` on the Slack line; an account linked before the change still shows its member id. - API and web unit tests cover the stored name and the fallback.

Slack Connect stores the sender's users.info display name (new users:read scope); Profile/Users Slack line shows @name, falls back to member id. API+web tests, quick gate green, visual check OK; PR #6691 queued. Older installs need reinstall for names. Memory: none applied.