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/__init__.py

@@ -17,6 +17,8 @@ from .architecture_scan import (
 from .base import Base
 from .company import Company
 from .content_page import ContentPage
+from .platform import Platform
+from .vendor_platform import VendorPlatform
 from .customer import Customer, CustomerAddress
 from .password_reset_token import PasswordResetToken
 from .email import EmailCategory, EmailLog, EmailStatus, EmailTemplate
@@ -106,6 +108,9 @@ __all__ = [
     "VendorTheme",
     # Content
     "ContentPage",
+    # Platform
+    "Platform",
+    "VendorPlatform",
     # Customer & Auth
     "Customer",
     "CustomerAddress",