fix: implement handleSort method for sortable table headers

- Update th_sortable macro to call handleSort(key) instead of inline logic
- Add handleSort method to subscriptions.js, subscription-tiers.js, billing-history.js
- Add sort_by and sort_order params to API calls in all three files
- Reset to page 1 when sort changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/admin/js/subscriptions.js

@@ -144,6 +144,8 @@ function adminSubscriptions() {
                 if (this.filters.status) params.append('status', this.filters.status);
                 if (this.filters.tier) params.append('tier', this.filters.tier);
                 if (this.filters.search) params.append('search', this.filters.search);
+                if (this.sortBy) params.append('sort_by', this.sortBy);
+                if (this.sortOrder) params.append('sort_order', this.sortOrder);
 
                 const data = await apiClient.get(`/admin/subscriptions?${params}`);
                 this.subscriptions = data.subscriptions || [];
@@ -168,6 +170,17 @@ function adminSubscriptions() {
             this.loadSubscriptions();
         },
 
+        handleSort(key) {
+            if (this.sortBy === key) {
+                this.sortOrder = this.sortOrder === 'asc' ? 'desc' : 'asc';
+            } else {
+                this.sortBy = key;
+                this.sortOrder = 'asc';
+            }
+            this.pagination.page = 1;
+            this.loadSubscriptions();
+        },
+
         previousPage() {
             if (this.pagination.page > 1) {
                 this.pagination.page--;