feat(members): add taxReceiptPreferences schema field (Phase 1 forward-compat)

Nested object with filesCanadianTaxes, middleInitial, confirmedAddress
(street/city/province/country/postalCode), setupCompletedAt. All
default to null/false so existing members read as 'not set up'.

Schema-only: no Zod, no API route, no UI. Phase 2 will build the
account-page preferences flow and bind to these fields without
needing a migration.

server/models/member.js

@@ -95,6 +95,19 @@ const memberSchema = new mongoose.Schema({
     acceptedAt: Date,
   },
 
+  taxReceiptPreferences: {
+    filesCanadianTaxes: { type: Boolean, default: false },
+    middleInitial: { type: String, default: null },
+    confirmedAddress: {
+      street: { type: String, default: null },
+      city: { type: String, default: null },
+      province: { type: String, default: null },
+      country: { type: String, default: null },
+      postalCode: { type: String, default: null },
+    },
+    setupCompletedAt: { type: Date, default: null },
+  },
+
   // Magic link single-use enforcement
   magicLinkJti: String,
   magicLinkJtiUsed: { type: Boolean, default: false },