Replaces the per-file inviteToSlack helpers with a single auto-flag
call. Self-serve activation paths now check for pre-existing workspace
membership (silent on miss) instead of attempting an admin-only invite.
- helcim/subscription.post.js: removed local inviteToSlack; both
free- and paid-tier activation branches now call the helper, then
notifyNewMember with the canonical 'manual_invitation_required' arg.
- members/create.post.js: same shape — helper + canonical notify arg.
- invite/accept.post.js (free-tier branch): added the helper call after
member creation. Free-tier had no prior Slack call (audit confirmed);
paid-tier remains untouched and activates via the Helcim webhook.
Admin-created and CSV-imported members intentionally do NOT call the
helper — admins flip the flag manually after sending the invite.
Test stub for autoFlagPreExistingSlackAccess added to server setup.
