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/billing-history.js

@@ -136,6 +136,8 @@ function adminBillingHistory() {
                 params.append('per_page', this.pagination.per_page);
                 if (this.filters.vendor_id) params.append('vendor_id', this.filters.vendor_id);
                 if (this.filters.status) params.append('status', this.filters.status);
+                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/billing/history?${params}`);
                 this.invoices = data.invoices || [];
@@ -154,6 +156,17 @@ function adminBillingHistory() {
             }
         },
 
+        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.loadInvoices();
+        },
+
         updateStatusCounts() {
             // Reset counts
             this.statusCounts = {