feat(admin): PATCH /api/admin/members/:id/slack-status

Endpoint that flips a member's slackInvited flag manually after the
admin has actually sent the Slack invitation through Slack's UI. No
Slack API call is made from this app.

- Body validated via Zod literal-true schema (no undo path for the
  pilot — admins correct mistakes in the database if needed).
- Idempotent: re-marking an already-invited member is a no-op,
  preserving the original slackInvitedAt and not duplicating the
  activity log entry.
- Activity log: slack_invited_manually, actor = admin from
  requireAdmin, subject = the target member.

server/utils/schemas.js

@@ -314,6 +314,10 @@ export const adminRoleUpdateSchema = z.object({
   role: z.enum(['admin', 'member'])
 })
 
+export const adminSlackStatusSchema = z.object({
+  slackInvited: z.literal(true)
+}).strict()
+
 export const bulkMemberImportSchema = z.object({
   members: z.array(z.object({
     name: z.string().min(1).max(200),