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].put.js

@@ -0,0 +1,26 @@
+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')
+  const body = await validateBody(event, preRegistrantStatusUpdateSchema)
+  await connectDB()
+
+  const existing = await PreRegistration.findById(id)
+  if (!existing) {
+    throw createError({ statusCode: 404, statusMessage: 'Pre-registrant not found' })
+  }
+
+  // Only allow status changes to pending/selected (invite/accept are handled by other endpoints)
+  if (existing.status === 'accepted') {
+    throw createError({ statusCode: 400, statusMessage: 'Cannot modify an accepted pre-registrant' })
+  }
+
+  const update = {}
+  if (body.status !== undefined) update.status = body.status
+  if (body.adminNotes !== undefined) update.adminNotes = body.adminNotes
+
+  const updated = await PreRegistration.findByIdAndUpdate(id, update, { new: true }).lean()
+  return updated
+})