refactor: remove backward compatibility code for pre-launch baseline

Clean up accumulated backward-compat shims, deprecated wrappers, unused
aliases, and legacy code across the codebase. Since the platform is not
live yet, this establishes a clean baseline.

Changes:
- Delete deprecated middleware/context.py (RequestContext, get_request_context)
- Remove unused factory get_store_email_settings_service()
- Remove deprecated pagination_full macro, /admin/platform-homepage route
- Remove ConversationResponse, InvoiceSettings* unprefixed aliases
- Simplify celery_config.py (remove empty LEGACY_TASK_MODULES)
- Standardize billing exceptions: *Error aliases → *Exception names
- Consolidate duplicate TierNotFoundError/FeatureNotFoundError classes
- Remove deprecated is_admin_request() from Store/PlatformContextManager
- Remove is_platform_default field, MediaUploadResponse legacy flat fields
- Remove MediaItemResponse.url alias, update JS to use file_url
- Update all affected tests and documentation

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

app/modules/cms/static/shared/js/media-picker.js

@@ -262,9 +262,9 @@ function mediaPickerMixin(storeIdGetter, multiSelect = false) {
          */
         setMainImage(media) {
             if (this.form) {
-                this.form.primary_image_url = media.url;
+                this.form.primary_image_url = media.file_url;
             }
-            mediaPickerLog.info('Main image set:', media.url);
+            mediaPickerLog.info('Main image set:', media.file_url);
         },
 
         /**
@@ -272,13 +272,13 @@ function mediaPickerMixin(storeIdGetter, multiSelect = false) {
          */
         addAdditionalImages(mediaList) {
             if (this.form && Array.isArray(this.form.additional_images)) {
-                const newUrls = mediaList.map(m => m.url);
+                const newUrls = mediaList.map(m => m.file_url);
                 this.form.additional_images = [
                     ...this.form.additional_images,
                     ...newUrls
                 ];
             }
-            mediaPickerLog.info('Additional images added:', mediaList.map(m => m.url));
+            mediaPickerLog.info('Additional images added:', mediaList.map(m => m.file_url));
         },
 
         /**

app/modules/cms/static/store/js/content-pages.js

@@ -73,7 +73,7 @@ function storeContentPagesManager() {
 
                 // Platform pages - filter to only show actual platform defaults
                 const allPages = platformResponse.data || platformResponse || [];
-                this.platformPages = allPages.filter(p => p.is_platform_default);
+                this.platformPages = allPages.filter(p => p.is_platform_page);
 
                 // Store's custom pages (includes overrides)
                 this.customPages = storeResponse.data || storeResponse || [];