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.

dist/routes/billing.js

@@ -65,7 +65,7 @@ a { color: #4f9; }
 <div class="card">
 <h1>🎉 Welcome to Pro!</h1>
 <p>Your API key:</p>
-<div class="key" onclick="navigator.clipboard.writeText('${escapeHtml(keyInfo.key)}')" title="Click to copy">${escapeHtml(keyInfo.key)}</div>
+<div class="key" style="position:relative">${escapeHtml(keyInfo.key)}<button onclick="navigator.clipboard.writeText('${escapeHtml(keyInfo.key)}');this.textContent='Copied!';setTimeout(()=>this.textContent='Copy',1500)" style="position:absolute;top:8px;right:8px;background:#4f9;color:#0a0a0a;border:none;border-radius:4px;padding:4px 12px;cursor:pointer;font-size:0.8rem;font-family:system-ui">Copy</button></div>
 <p><strong>Save this key!</strong> It won't be shown again.</p>
 <p>10,000 PDFs/month • All endpoints • Priority support</p>
 <p><a href="/docs">View API docs →</a></p>