feat(activation): wire autoFlagPreExistingSlackAccess into self-serve paths

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.

server/api/helcim/subscription.post.js

@@ -8,77 +8,6 @@ import { createHelcimSubscription, generateIdempotencyKey, listHelcimCustomerTra
 import { sendWelcomeEmail } from '../../utils/resend.js'
 import { upsertPaymentFromHelcim } from '../../utils/payments.js'
 
-// Function to invite member to Slack
-async function inviteToSlack(member) {
-  try {
-    const slackService = getSlackService()
-    if (!slackService) {
-      console.warn('Slack service not configured, skipping invitation')
-      return
-    }
-
-    console.log(`Processing Slack invitation for ${member.email}...`)
-    
-    const inviteResult = await slackService.inviteUserToSlack(
-      member.email,
-      member.name
-    )
-
-    if (inviteResult.success) {
-      const update = {}
-      if (inviteResult.status === 'existing_user_added_to_channel' ||
-          inviteResult.status === 'user_already_in_channel' ||
-          inviteResult.status === 'new_user_invited_to_workspace') {
-        update.slackInviteStatus = 'sent'
-        update.slackUserId = inviteResult.userId
-        update.slackInvited = true
-      } else {
-        update.slackInviteStatus = 'pending'
-        update.slackInvited = false
-      }
-      await Member.findByIdAndUpdate(
-        member._id,
-        { $set: update },
-        { runValidators: false }
-      )
-
-      // Send notification to vetting channel
-      await slackService.notifyNewMember(
-        member.name,
-        member.email,
-        member.circle,
-        member.contributionAmount,
-        inviteResult.status
-      )
-
-      console.log(`Successfully processed Slack invitation for ${member.email}: ${inviteResult.status}`)
-    } else {
-      await Member.findByIdAndUpdate(
-        member._id,
-        { $set: { slackInviteStatus: 'failed' } },
-        { runValidators: false }
-      )
-
-      console.error(`Failed to process Slack invitation for ${member.email}: ${inviteResult.error}`)
-      // Don't throw error - subscription creation should still succeed
-    }
-  } catch (error) {
-    console.error('Error during Slack invitation process:', error)
-
-    try {
-      await Member.findByIdAndUpdate(
-        member._id,
-        { $set: { slackInviteStatus: 'failed' } },
-        { runValidators: false }
-      )
-    } catch (saveError) {
-      console.error('Failed to update member Slack status:', saveError)
-    }
-    
-    // Don't throw error - subscription creation should still succeed
-  }
-}
-
 export default defineEventHandler(async (event) => {
   try {
     // Membership signup completes subscription before email verify; allow the
@@ -109,7 +38,21 @@ export default defineEventHandler(async (event) => {
 
       logActivity(member._id, 'subscription_created', { amount: body.contributionAmount })
 
-      await inviteToSlack(member)
+      await autoFlagPreExistingSlackAccess(member)
+      try {
+        const slackService = getSlackService()
+        if (slackService) {
+          await slackService.notifyNewMember(
+            member.name,
+            member.email,
+            member.circle,
+            member.contributionAmount,
+            'manual_invitation_required'
+          )
+        }
+      } catch (err) {
+        console.error('[slack] notifyNewMember failed:', err)
+      }
       if (isFirstActivation) await sendWelcomeEmail(member)
 
       return {
@@ -207,7 +150,21 @@ export default defineEventHandler(async (event) => {
       console.error('[payments] initial charge log failed, will be picked up by reconciliation:', err?.message || err)
     }
 
-    await inviteToSlack(member)
+    await autoFlagPreExistingSlackAccess(member)
+    try {
+      const slackService = getSlackService()
+      if (slackService) {
+        await slackService.notifyNewMember(
+          member.name,
+          member.email,
+          member.circle,
+          member.contributionAmount,
+          'manual_invitation_required'
+        )
+      }
+    } catch (err) {
+      console.error('[slack] notifyNewMember failed:', err)
+    }
     if (isFirstActivation) await sendWelcomeEmail(member)
 
     return {