feat: add DB Explorer for read-only SQLite inspection

Adds /db-explorer page and JSON API to browse all 15 SQLite databases
in data/. Sidebar lists databases with sizes, clicking one renders all
tables as scrollable data tables with row truncation at 200.

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

src/dashboard/templates/base.html

@@ -78,6 +78,7 @@
           <a href="/swarm/events" class="mc-test-link">EVENTS</a>
           <a href="/router/status" class="mc-test-link">ROUTER</a>
           <a href="/grok/status" class="mc-test-link mc-link-grok">GROK</a>
+          <a href="/db-explorer" class="mc-test-link">DB EXPLORER</a>
         </div>
       </div>
       <div class="mc-nav-dropdown">
@@ -137,6 +138,7 @@
     <a href="/swarm/events" class="mc-mobile-link">EVENTS</a>
     <a href="/router/status" class="mc-mobile-link">ROUTER</a>
     <a href="/grok/status" class="mc-mobile-link">GROK</a>
+    <a href="/db-explorer" class="mc-mobile-link">DB EXPLORER</a>
     <div class="mc-mobile-section-label">COMMERCE</div>
     <a href="/lightning/ledger" class="mc-mobile-link">LEDGER</a>
     <a href="/creative/ui" class="mc-mobile-link">CREATIVE</a>