refactor(member): replace slackInviteStatus with slackInvitedAt

Schema change for wave-based Slack onboarding. The string enum
slackInviteStatus is replaced with a simple slackInvitedAt: Date —
boolean slackInvited is the source of truth, the date records when.

Call sites that flip slackInvited:true must stamp slackInvitedAt
in the same update (no pre-save hook, per findByIdAndUpdate convention).

Sweeps of remaining slackInviteStatus references land in later tasks.

server/models/member.js

@@ -59,11 +59,7 @@ const memberSchema = new mongoose.Schema({
   nextBillingDate: Date,
   lastCancelledAt: Date,
   slackInvited: { type: Boolean, default: false },
-  slackInviteStatus: {
-    type: String,
-    enum: ["pending", "sent", "failed", "accepted", "joined"],
-    default: "pending",
-  },
+  slackInvitedAt: { type: Date },
   slackUserId: String,
 
   // Profile fields