fix: BUG-046 usage endpoint data leak, BUG-047 copy button, BUG-048 email change links

BUG-046 (CRITICAL): getUsageStats() now accepts apiKey param and returns
only that key usage instead of all users. Route passes req.apiKeyInfo.key.

BUG-047: Added visible Copy button to Pro key success page in billing.ts.

BUG-048: Added class="open-email-change" to Change Email links in all
HTML pages so the JS modal opener can find them.

public/terms.html

@@ -264,7 +264,7 @@ footer .container { display: flex; align-items: center; justify-content: space-b
       <a href="/">Home</a>
       <a href="/docs">Docs</a>
       <a href="/health">API Status</a>
-      <a href="/#change-email">Change Email</a>
+      <a href="/#change-email" class="open-email-change">Change Email</a>
       <a href="/impressum">Impressum</a>
       <a href="/privacy">Privacy Policy</a>
       <a href="/terms">Terms of Service</a>