feat: add multi-platform CMS architecture (Phase 1)

Implement the foundation for multi-platform support allowing independent
business offerings (OMS, Loyalty, etc.) with their own CMS pages.

Database Models:
- Add Platform model for business offerings (domain, branding, config)
- Add VendorPlatform junction table for many-to-many relationship
- Update SubscriptionTier with platform_id and CMS limits
- Update ContentPage with platform_id, is_platform_page for three-tier hierarchy
- Add CMS feature codes (cms_basic, cms_custom_pages, cms_templates, etc.)

Three-Tier Content Resolution:
1. Vendor override (platform_id + vendor_id + slug)
2. Vendor default (platform_id + vendor_id=NULL + is_platform_page=False)
3. Platform marketing pages (is_platform_page=True)

New Components:
- PlatformContextMiddleware for detecting platform from domain/path
- ContentPageService updated with full three-tier resolution
- Platform folder structure (app/platforms/oms/, app/platforms/loyalty/)
- Alembic migration with backfill for existing data

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

models/database/feature.py

@@ -33,6 +33,7 @@ class FeatureCategory(str, enum.Enum):
     TEAM = "team"
     BRANDING = "branding"
     CUSTOMERS = "customers"
+    CMS = "cms"
 
 
 class FeatureUILocation(str, enum.Enum):
@@ -189,3 +190,11 @@ class FeatureCode:
     CUSTOMER_VIEW = "customer_view"
     CUSTOMER_EXPORT = "customer_export"
     CUSTOMER_MESSAGING = "customer_messaging"
+
+    # CMS
+    CMS_BASIC = "cms_basic"  # Basic CMS functionality (override defaults)
+    CMS_CUSTOM_PAGES = "cms_custom_pages"  # Create custom pages beyond defaults
+    CMS_UNLIMITED_PAGES = "cms_unlimited_pages"  # No page limit
+    CMS_TEMPLATES = "cms_templates"  # Access to page templates
+    CMS_SEO = "cms_seo"  # Advanced SEO features
+    CMS_SCHEDULING = "cms_scheduling"  # Schedule page publish/unpublish