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>

app/platforms/loyalty/__init__.py

@@ -0,0 +1,10 @@
+# app/platforms/loyalty/__init__.py
+"""
+Loyalty Platform
+
+Platform for customer loyalty programs and rewards.
+"""
+
+from .config import LoyaltyPlatformConfig
+
+__all__ = ["LoyaltyPlatformConfig"]

app/platforms/loyalty/config.py

@@ -0,0 +1,49 @@
+# app/platforms/loyalty/config.py
+"""
+Loyalty Platform Configuration
+
+Configuration for the Loyalty/Rewards platform.
+"""
+
+from app.platforms.shared.base_platform import BasePlatformConfig
+
+
+class LoyaltyPlatformConfig(BasePlatformConfig):
+    """Configuration for the Loyalty platform."""
+
+    @property
+    def code(self) -> str:
+        return "loyalty"
+
+    @property
+    def name(self) -> str:
+        return "Loyalty+"
+
+    @property
+    def description(self) -> str:
+        return "Customer loyalty and rewards platform"
+
+    @property
+    def features(self) -> list[str]:
+        """Loyalty-specific features."""
+        return [
+            "loyalty_points",
+            "rewards_catalog",
+            "customer_tiers",
+            "referral_program",
+        ]
+
+    @property
+    def vendor_default_page_slugs(self) -> list[str]:
+        """Default pages for Loyalty vendor storefronts."""
+        return [
+            "about",
+            "how-it-works",
+            "rewards",
+            "terms-of-service",
+            "privacy-policy",
+        ]
+
+
+# Singleton instance
+loyalty_config = LoyaltyPlatformConfig()

app/platforms/loyalty/routes/__init__.py

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+# app/platforms/loyalty/routes/__init__.py
+"""Loyalty platform routes."""

app/platforms/loyalty/templates/__init__.py

@@ -0,0 +1,2 @@
+# app/platforms/loyalty/templates/__init__.py
+"""Loyalty platform templates."""