feat: tree view for cached objects, top-files stats on dashboard (#48)

- Objects page renders paths as a collapsible tree instead of flat list
  with expand/collapse all, aggregated size/hits per directory
- Dashboard gains top-files-by-hits and top-files-by-bandwidth tables
- Backend: new /api/v2/stats/top-files-by-hits and
  /api/v2/stats/top-files-by-bandwidth endpoints
- Raised per_page max to 5000 for objects listing

---------

Co-authored-by: Ben Vincent <ben@unkin.net>
Reviewed-on: #48

ui/src/pages/Dashboard.css

@@ -45,3 +45,38 @@
   font-size: 0.9em;
   padding: 32px 0;
 }
+
+.top-files-grid {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 24px;
+  margin-top: 8px;
+}
+
+@media (max-width: 1100px) {
+  .top-files-grid {
+    grid-template-columns: 1fr;
+  }
+}
+
+.file-link {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  text-decoration: none;
+  color: inherit;
+}
+
+.file-link:hover .file-path {
+  text-decoration: underline;
+}
+
+.file-remote {
+  font-size: 0.75em;
+  color: var(--text-muted);
+}
+
+.file-path {
+  font-size: 0.82em;
+  word-break: break-all;
+}