feat: dynamic merchant sidebar with module-driven menus

Replace the hardcoded merchant sidebar with a dynamic menu system driven
by module definitions, matching the existing admin frontend pattern.
Modules declare FrontendType.MERCHANT menus in their definition.py, and
a new API endpoint unions enabled modules across all platforms the
merchant is subscribed to — so loyalty only appears when enabled.

- Add MERCHANT menu definitions to core, billing, tenancy, loyalty modules
- Extend MenuDiscoveryService with enabled_module_codes parameter
- Create GET /merchants/core/menu/render/merchant endpoint
- Update merchant Alpine.js with loadMenuConfig() and dynamic section state
- Replace hardcoded sidebar.html with x-for rendering + loading skeleton + fallback
- Add 36 unit and integration tests for menu discovery, service, and endpoint

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

app/modules/core/static/merchant/js/init-alpine.js

@@ -21,11 +21,8 @@ function getMerchantSidebarSectionsFromStorage() {
     } catch (e) {
         console.warn('[MERCHANT INIT-ALPINE] Failed to load sidebar state from localStorage:', e);
     }
-    // Default: all sections open
-    return {
-        billing: true,
-        account: true
-    };
+    // Default: empty — dynamic menu will initialize section state
+    return {};
 }
 
 function saveMerchantSidebarSectionsToStorage(sections) {
@@ -36,6 +33,19 @@ function saveMerchantSidebarSectionsToStorage(sections) {
     }
 }
 
+// Helper to find section ID for a page from menu data
+function findSectionForPage(menuData, pageId) {
+    if (!menuData?.sections) return null;
+    for (const section of menuData.sections) {
+        for (const item of (section.items || [])) {
+            if (item.id === pageId) {
+                return section.id;
+            }
+        }
+    }
+    return null;
+}
+
 function data() {
     console.log('[MERCHANT INIT-ALPINE] data() function called');
     return {
@@ -48,6 +58,10 @@ function data() {
         // Sidebar collapsible sections state
         openSections: getMerchantSidebarSectionsFromStorage(),
 
+        // Dynamic menu (loaded from API)
+        menuData: null,
+        menuLoading: false,
+
         init() {
             // Set current page from URL
             const path = window.location.pathname;
@@ -87,6 +101,9 @@ function data() {
                     // Ignore storage errors
                 }
             }
+
+            // Load dynamic menu
+            this.loadMenuConfig();
         },
 
         toggleSideMenu() {
@@ -116,6 +133,54 @@ function data() {
             saveMerchantSidebarSectionsToStorage(this.openSections);
         },
 
+        // Auto-expand section containing current page
+        expandSectionForCurrentPage() {
+            if (!this.menuData) return;
+            const section = findSectionForPage(this.menuData, this.currentPage);
+            if (section && !this.openSections[section]) {
+                this.openSections[section] = true;
+                saveMerchantSidebarSectionsToStorage(this.openSections);
+            }
+        },
+
+        // Dynamic menu loading from API
+        async loadMenuConfig() {
+            if (this.menuData || this.menuLoading) return;
+
+            // Skip if apiClient is not available (e.g., on login page)
+            if (typeof apiClient === 'undefined') {
+                console.debug('Menu config: apiClient not available');
+                return;
+            }
+
+            // Skip if not authenticated
+            if (!localStorage.getItem('merchant_token')) {
+                console.debug('Menu config: no merchant_token, skipping');
+                return;
+            }
+
+            this.menuLoading = true;
+            try {
+                this.menuData = await apiClient.get('/merchants/core/menu/render/merchant');
+                const sections = this.menuData?.sections || [];
+                for (const section of sections) {
+                    // Initialize openSections for new sections (default: open)
+                    if (this.openSections[section.id] === undefined) {
+                        this.openSections[section.id] = true;
+                    }
+                }
+                saveMerchantSidebarSectionsToStorage(this.openSections);
+                console.debug('Menu config loaded:', sections.length, 'sections');
+
+                // Auto-expand section containing current page
+                this.expandSectionForCurrentPage();
+            } catch (e) {
+                console.debug('Menu config not loaded, using fallback:', e?.message || e);
+            } finally {
+                this.menuLoading = false;
+            }
+        },
+
         async handleLogout() {
             console.log('Logging out merchant user...');