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/shared/base_platform.py

@@ -0,0 +1,97 @@
+# app/platforms/shared/base_platform.py
+"""
+Base Platform Configuration
+
+Provides a base class for platform-specific configurations.
+Each platform (OMS, Loyalty, etc.) should extend this class.
+"""
+
+from abc import ABC, abstractmethod
+from typing import Any
+
+
+class BasePlatformConfig(ABC):
+    """
+    Base configuration class for platforms.
+
+    Each platform should create a config.py that extends this class
+    and provides platform-specific settings.
+    """
+
+    @property
+    @abstractmethod
+    def code(self) -> str:
+        """Unique platform code (e.g., 'oms', 'loyalty')."""
+        pass
+
+    @property
+    @abstractmethod
+    def name(self) -> str:
+        """Display name (e.g., 'Wizamart OMS')."""
+        pass
+
+    @property
+    def description(self) -> str:
+        """Platform description."""
+        return ""
+
+    @property
+    def default_language(self) -> str:
+        """Default language code."""
+        return "fr"
+
+    @property
+    def supported_languages(self) -> list[str]:
+        """List of supported language codes."""
+        return ["fr", "de", "en"]
+
+    @property
+    def theme_defaults(self) -> dict[str, Any]:
+        """Default theme configuration."""
+        return {
+            "primary_color": "#6366f1",
+            "secondary_color": "#8b5cf6",
+            "font_family": "Inter, sans-serif",
+        }
+
+    @property
+    def features(self) -> list[str]:
+        """List of feature codes enabled for this platform."""
+        return []
+
+    @property
+    def marketing_page_slugs(self) -> list[str]:
+        """
+        Slugs that should be treated as platform marketing pages.
+
+        These pages describe the platform itself (pricing, features, etc.)
+        rather than being vendor storefront content.
+        """
+        return [
+            "home",
+            "platform_homepage",
+            "pricing",
+            "about",
+            "contact",
+            "faq",
+            "terms",
+            "privacy",
+            "features",
+            "integrations",
+        ]
+
+    @property
+    def vendor_default_page_slugs(self) -> list[str]:
+        """
+        Slugs for default vendor storefront pages.
+
+        These pages provide fallback content for vendors who haven't
+        customized their storefront.
+        """
+        return [
+            "about",
+            "shipping",
+            "returns",
+            "privacy-policy",
+            "terms-of-service",
+        ]