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/vendor.py

@@ -245,6 +245,13 @@ class Vendor(Base, TimestampMixin):
         cascade="all, delete-orphan",
     )
 
+    # Platform memberships (many-to-many via junction table)
+    vendor_platforms = relationship(
+        "VendorPlatform",
+        back_populates="vendor",
+        cascade="all, delete-orphan",
+    )
+
     def __repr__(self):
         """String representation of the Vendor object."""
         return f"<Vendor(id={self.id}, vendor_code='{self.vendor_code}', name='{self.name}', subdomain='{self.subdomain}')>"