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>

middleware/__init__.py

@@ -1 +1,28 @@
+# middleware/__init__.py
+"""Middleware package for request processing."""
 
+from .platform_context import (
+    PlatformContextManager,
+    PlatformContextMiddleware,
+    get_current_platform,
+    require_platform_context,
+)
+from .vendor_context import (
+    VendorContextManager,
+    VendorContextMiddleware,
+    get_current_vendor,
+    require_vendor_context,
+)
+
+__all__ = [
+    # Platform context
+    "PlatformContextManager",
+    "PlatformContextMiddleware",
+    "get_current_platform",
+    "require_platform_context",
+    # Vendor context
+    "VendorContextManager",
+    "VendorContextMiddleware",
+    "get_current_vendor",
+    "require_vendor_context",
+]