feat: Add Flush Cache tool page to web UI

Adds a /flush-cache web page with a button to clear all cached Google
Sheets data and reset refresh timers. Link added to Tools nav across
all templates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/payments.html

@@ -199,6 +199,7 @@
         <div class="nav-archived">
             <span style="color: #666; margin-right: 5px;">Tools:</span>
             <a href="/sync-bank">[Sync Bank Data]</a>
+            <a href="/flush-cache">[Flush Cache]</a>
         </div>
     </div>