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/[id].get.js

@@ -0,0 +1,15 @@
+import PreRegistration from '../../../models/preRegistration.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+  const id = getRouterParam(event, 'id')
+  await connectDB()
+
+  const preRegistrant = await PreRegistration.findById(id).lean()
+  if (!preRegistrant) {
+    throw createError({ statusCode: 404, statusMessage: 'Pre-registrant not found' })
+  }
+
+  return preRegistrant
+})