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/routes/api/merchant.py

@@ -12,6 +12,7 @@ Aggregates:
 from fastapi import APIRouter
 
 from .merchant_dashboard import merchant_dashboard_router
+from .merchant_menu import merchant_menu_router
 
 ROUTE_CONFIG = {
     "prefix": "/core",
@@ -20,3 +21,4 @@ ROUTE_CONFIG = {
 router = APIRouter()
 
 router.include_router(merchant_dashboard_router, tags=["merchant-dashboard"])
+router.include_router(merchant_menu_router, tags=["merchant-menu"])

app/modules/core/routes/api/merchant_menu.py

@@ -0,0 +1,141 @@
+# app/modules/core/routes/api/merchant_menu.py
+"""
+Merchant menu rendering endpoint.
+
+Provides the dynamic sidebar menu for the merchant portal:
+- GET /menu/render/merchant - Get rendered merchant menu for current user
+
+Menu sections are driven by module definitions (FrontendType.MERCHANT).
+Only modules enabled on platforms the merchant is actively subscribed to
+will appear in the sidebar.
+"""
+
+import logging
+from typing import Any
+
+from fastapi import APIRouter, Depends, Request
+from pydantic import BaseModel
+from sqlalchemy.orm import Session
+
+from app.api.deps import get_current_merchant_from_cookie_or_header, get_db
+from app.modules.core.services.menu_service import menu_service
+from app.modules.enums import FrontendType
+from app.utils.i18n import translate
+from models.schema.auth import UserContext
+
+logger = logging.getLogger(__name__)
+
+merchant_menu_router = APIRouter(prefix="/menu")
+
+
+# =============================================================================
+# Schemas
+# =============================================================================
+
+
+class MenuSectionResponse(BaseModel):
+    """Menu section for rendering."""
+
+    id: str
+    label: str | None = None
+    items: list[dict[str, Any]]
+
+
+class RenderedMenuResponse(BaseModel):
+    """Rendered menu for frontend."""
+
+    frontend_type: str
+    sections: list[MenuSectionResponse]
+
+
+# =============================================================================
+# Helpers
+# =============================================================================
+
+
+def _translate_label(label_key: str | None, language: str) -> str | None:
+    """Translate a label key, falling back to a readable version of the key."""
+    if not label_key:
+        return None
+
+    translated = translate(label_key, language=language)
+
+    # If translation returned the key itself, create a readable fallback
+    if translated == label_key:
+        parts = label_key.split(".")
+        last_part = parts[-1]
+        return last_part.replace("_", " ").title()
+
+    return translated
+
+
+# =============================================================================
+# Endpoint
+# =============================================================================
+
+
+@merchant_menu_router.get("/render/merchant", response_model=RenderedMenuResponse)
+async def get_rendered_merchant_menu(
+    request: Request,
+    db: Session = Depends(get_db),
+    current_user: UserContext = Depends(get_current_merchant_from_cookie_or_header),
+):
+    """
+    Get the rendered merchant menu for the current user.
+
+    Returns the filtered menu structure based on modules enabled
+    on platforms the merchant is subscribed to.
+
+    Used by the merchant frontend to render the sidebar dynamically.
+    """
+    # Resolve the merchant for this user (via service layer)
+    merchant = menu_service.get_merchant_for_menu(db, current_user.id)
+
+    if not merchant:
+        # No merchant found — return empty menu
+        return RenderedMenuResponse(
+            frontend_type=FrontendType.MERCHANT.value,
+            sections=[],
+        )
+
+    # Get union of enabled module codes across all subscribed platforms
+    enabled_codes = menu_service.get_merchant_enabled_module_codes(db, merchant.id)
+
+    # Get filtered menu using enabled_module_codes override
+    menu = menu_service.get_menu_for_rendering(
+        db=db,
+        frontend_type=FrontendType.MERCHANT,
+        enabled_module_codes=enabled_codes,
+    )
+
+    # Resolve language
+    language = current_user.preferred_language or getattr(
+        request.state, "language", "en"
+    )
+
+    # Translate section and item labels
+    sections = []
+    for section in menu:
+        translated_items = []
+        for item in section.items:
+            translated_items.append(
+                {
+                    "id": item.id,
+                    "label": _translate_label(item.label_key, language),
+                    "icon": item.icon,
+                    "url": item.route,
+                }
+            )
+
+        sections.append(
+            MenuSectionResponse(
+                id=section.id,
+                label=_translate_label(section.label_key, language),
+                items=translated_items,
+            )
+        )
+
+    return RenderedMenuResponse(
+        frontend_type=FrontendType.MERCHANT.value,
+        sections=sections,
+    )