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/bulk-status.patch.js

@@ -0,0 +1,16 @@
+import PreRegistration from '../../../models/preRegistration.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+  const { ids, status } = await validateBody(event, preRegistrantBulkStatusSchema)
+  await connectDB()
+
+  // Only update pre-registrants that aren't already accepted
+  const result = await PreRegistration.updateMany(
+    { _id: { $in: ids }, status: { $nin: ['accepted'] } },
+    { $set: { status } }
+  )
+
+  return { modified: result.modifiedCount, total: ids.length }
+})