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/tenancy/definition.py

@@ -91,6 +91,10 @@ tenancy_module = ModuleDefinition(
         FrontendType.STORE: [
             "team",
         ],
+        FrontendType.MERCHANT: [
+            "stores",
+            "profile",
+        ],
     },
     # New module-driven menu definitions
     menus={
@@ -160,6 +164,30 @@ tenancy_module = ModuleDefinition(
                 ],
             ),
         ],
+        FrontendType.MERCHANT: [
+            MenuSectionDefinition(
+                id="account",
+                label_key="tenancy.menu.account",
+                icon="cog",
+                order=900,
+                items=[
+                    MenuItemDefinition(
+                        id="stores",
+                        label_key="tenancy.menu.stores",
+                        icon="shopping-bag",
+                        route="/merchants/account/stores",
+                        order=10,
+                    ),
+                    MenuItemDefinition(
+                        id="profile",
+                        label_key="tenancy.menu.profile",
+                        icon="user",
+                        route="/merchants/account/profile",
+                        order=20,
+                    ),
+                ],
+            ),
+        ],
         FrontendType.STORE: [
             MenuSectionDefinition(
                 id="account",