feat: add admin menu configuration and sidebar improvements

- Add AdminMenuConfig model for per-platform menu customization
- Add menu registry for centralized menu configuration
- Add my-menu-config and platform-menu-config admin pages
- Update sidebar with improved layout and Alpine.js interactions
- Add FrontendType enum for admin/vendor menu separation
- Document self-contained module patterns in session note

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

app/services/platform_service.py

@@ -20,7 +20,7 @@ from sqlalchemy.orm import Session
 from app.exceptions.platform import (
     PlatformNotFoundException,
 )
-from models.database.content_page import ContentPage
+from app.modules.cms.models import ContentPage
 from models.database.platform import Platform
 from models.database.vendor_platform import VendorPlatform
 
@@ -81,6 +81,28 @@ class PlatformService:
         """
         return db.query(Platform).filter(Platform.code == code).first()
 
+    @staticmethod
+    def get_platform_by_id(db: Session, platform_id: int) -> Platform:
+        """
+        Get platform by ID.
+
+        Args:
+            db: Database session
+            platform_id: Platform ID
+
+        Returns:
+            Platform object
+
+        Raises:
+            PlatformNotFoundException: If platform not found
+        """
+        platform = db.query(Platform).filter(Platform.id == platform_id).first()
+
+        if not platform:
+            raise PlatformNotFoundException(str(platform_id))
+
+        return platform
+
     @staticmethod
     def list_platforms(
         db: Session, include_inactive: bool = False