Accept buyer phone in grant API and enrich existing users

payment-router now forwards name/phone from the order. Store phone on
create; for existing users fill name (when it is an email placeholder)
and phone (when empty) without overwriting meaningful data. Also stop
dropping the parsed phone column in CSV import.

src/app/admin/import-export/import-actions.ts

@@ -200,6 +200,7 @@ export async function applyImport(
           data: {
             name: row.name,
             email: row.email,
+            phone: row.phone || null,
             emailVerified: options.autoVerifyEmail,
             role: "student",
           },
@@ -231,10 +232,16 @@ export async function applyImport(
 
         created++;
       } else if (row.status === "update" && row.existingId) {
+        const existing = await prisma.user.findUnique({
+          where: { id: row.existingId },
+          select: { phone: true },
+        });
         await prisma.user.update({
           where: { id: row.existingId },
           data: {
             name: row.name || undefined,
+            // don't overwrite an already-filled phone
+            phone: existing?.phone ? undefined : row.phone || undefined,
           },
         });