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/index.get.js

@@ -0,0 +1,13 @@
+import PreRegistration from '../../../models/preRegistration.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+  await connectDB()
+
+  const preRegistrants = await PreRegistration.find()
+    .sort({ createdAt: -1 })
+    .lean()
+
+  return preRegistrants
+})