refactor(arch): Phase 2 — deduplicate isAdmin, convertToUsd, getBaseWalletType

- #54: Extract isAdmin() to src/middleware/auth.js, remove duplicates from 7 admin handlers
- #55: Add WalletUtils.convertToUsd(), replace 8 switch-case blocks across 4 files
- #56: Unify getBaseWalletType() — keep only WalletUtils version (most complete),
  remove duplicates from Wallet.js and userWalletsHandler.js

New file: src/middleware/auth.js
Net: -215 lines, +80 lines

Closes: #54, #55, #56

src/middleware/auth.js

@@ -0,0 +1,5 @@
+import config from '../config/config.js';
+
+export function isAdmin(userId) {
+  return config.ADMIN_IDS.includes(userId.toString());
+}