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/Objects.css

@@ -3,13 +3,58 @@
   align-items: center;
   gap: 12px;
   margin-bottom: 16px;
+  flex-wrap: wrap;
 }
 
-.obj-path {
+.tree-controls {
+  display: flex;
+  gap: 6px;
+  margin-left: auto;
+}
+
+.tree-table .tree-label {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+}
+
+.tree-toggle {
+  display: inline-block;
+  width: 16px;
+  font-size: 0.85em;
+  color: var(--text-muted);
+  user-select: none;
+  flex-shrink: 0;
+}
+
+.tree-dir-name {
+  font-weight: 600;
+  color: var(--text-bright);
+}
+
+.tree-file-name {
   font-size: 0.85em;
   word-break: break-all;
 }
 
+.tree-dir {
+  cursor: pointer;
+}
+
+.tree-dir:hover {
+  background: var(--bg-elevated);
+}
+
+.tree-summary {
+  font-size: 0.8em;
+  color: var(--text-muted);
+}
+
+.num-cell {
+  font-family: var(--font-mono);
+  font-size: 0.85em;
+}
+
 .hash-cell {
   font-size: 0.8em;
   color: var(--text-muted);
@@ -30,20 +75,6 @@
   background: rgba(239, 68, 68, 0.15);
 }
 
-.pagination {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  gap: 16px;
-  margin-top: 16px;
-}
-
-.page-info {
-  font-size: 0.85em;
-  color: var(--text-muted);
-  font-family: var(--font-mono);
-}
-
 .btn-sm {
   padding: 5px 12px;
   font-size: 0.85em;