feat: pre-registrant management and invitation system

Admin interface to review, filter, and batch-invite the 95 pre-registrants
from Baby Ghosts. Accept-invitation page pre-fills their data and collects
circle, pronouns, motivation, contribution tier, and agreement before
creating their member record.

server/api/admin/pre-registrants/stats.get.js

@@ -0,0 +1,19 @@
+import PreRegistration from '../../../models/preRegistration.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+  await connectDB()
+
+  const counts = await PreRegistration.aggregate([
+    { $group: { _id: '$status', count: { $sum: 1 } } }
+  ])
+
+  const stats = { total: 0, pending: 0, selected: 0, invited: 0, accepted: 0, expired: 0 }
+  for (const { _id, count } of counts) {
+    if (_id in stats) stats[_id] = count
+    stats.total += count
+  }
+
+  return stats
+})