feat: add platform detail/edit admin UI and service enhancements

- Add platform detail and edit admin pages with templates and JS
- Add ContentPageService methods: list_all_platform_pages, list_all_vendor_defaults
- Deprecate /admin/platform-homepage route (redirects to /admin/platforms)
- Add migration to fix content_page nullable columns
- Refine platform and vendor context middleware
- Add platform context middleware unit tests
- Update platforms.js with improved functionality
- Add section-based homepage plan documentation

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

app/services/content_page_service.py

@@ -305,6 +305,66 @@ class ContentPageService:
             .all()
         )
 
+    @staticmethod
+    def list_all_platform_pages(
+        db: Session,
+        include_unpublished: bool = False,
+    ) -> list[ContentPage]:
+        """
+        List all platform marketing pages across all platforms (for admin use).
+
+        Args:
+            db: Database session
+            include_unpublished: Include draft pages
+
+        Returns:
+            List of all platform marketing ContentPage objects
+        """
+        filters = [
+            ContentPage.vendor_id.is_(None),
+            ContentPage.is_platform_page.is_(True),
+        ]
+
+        if not include_unpublished:
+            filters.append(ContentPage.is_published.is_(True))
+
+        return (
+            db.query(ContentPage)
+            .filter(and_(*filters))
+            .order_by(ContentPage.platform_id, ContentPage.display_order, ContentPage.title)
+            .all()
+        )
+
+    @staticmethod
+    def list_all_vendor_defaults(
+        db: Session,
+        include_unpublished: bool = False,
+    ) -> list[ContentPage]:
+        """
+        List all vendor default pages across all platforms (for admin use).
+
+        Args:
+            db: Database session
+            include_unpublished: Include draft pages
+
+        Returns:
+            List of all vendor default ContentPage objects
+        """
+        filters = [
+            ContentPage.vendor_id.is_(None),
+            ContentPage.is_platform_page.is_(False),
+        ]
+
+        if not include_unpublished:
+            filters.append(ContentPage.is_published.is_(True))
+
+        return (
+            db.query(ContentPage)
+            .filter(and_(*filters))
+            .order_by(ContentPage.platform_id, ContentPage.display_order, ContentPage.title)
+            .all()
+        )
+
     # =========================================================================
     # CRUD Methods
     # =========================================================================