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/subscription.py

@@ -84,12 +84,26 @@ class SubscriptionTier(Base, TimestampMixin):
     Database-driven tier definitions with Stripe integration.
 
     Replaces the hardcoded TIER_LIMITS dict for dynamic tier management.
+
+    Can be:
+    - Global tier (platform_id=NULL): Available to all platforms
+    - Platform-specific tier (platform_id set): Only for that platform
     """
 
     __tablename__ = "subscription_tiers"
 
     id = Column(Integer, primary_key=True, index=True)
-    code = Column(String(30), unique=True, nullable=False, index=True)
+
+    # Platform association (NULL = global tier available to all platforms)
+    platform_id = Column(
+        Integer,
+        ForeignKey("platforms.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True,
+        comment="Platform this tier belongs to (NULL = global tier)",
+    )
+
+    code = Column(String(30), nullable=False, index=True)
     name = Column(String(100), nullable=False)
     description = Column(Text, nullable=True)
 
@@ -103,6 +117,18 @@ class SubscriptionTier(Base, TimestampMixin):
     team_members = Column(Integer, nullable=True)
     order_history_months = Column(Integer, nullable=True)
 
+    # CMS Limits (null = unlimited)
+    cms_pages_limit = Column(
+        Integer,
+        nullable=True,
+        comment="Total CMS pages limit (NULL = unlimited)",
+    )
+    cms_custom_pages_limit = Column(
+        Integer,
+        nullable=True,
+        comment="Custom pages limit, excluding overrides (NULL = unlimited)",
+    )
+
     # Features (JSON array of feature codes)
     features = Column(JSON, default=list)
 
@@ -116,8 +142,21 @@ class SubscriptionTier(Base, TimestampMixin):
     is_active = Column(Boolean, default=True, nullable=False)
     is_public = Column(Boolean, default=True, nullable=False)  # False for enterprise
 
+    # Relationship to Platform
+    platform = relationship(
+        "Platform",
+        back_populates="subscription_tiers",
+        foreign_keys=[platform_id],
+    )
+
+    # Unique constraint: tier code must be unique per platform (or globally if NULL)
+    __table_args__ = (
+        Index("idx_tier_platform_active", "platform_id", "is_active"),
+    )
+
     def __repr__(self):
-        return f"<SubscriptionTier(code='{self.code}', name='{self.name}')>"
+        platform_info = f", platform_id={self.platform_id}" if self.platform_id else ""
+        return f"<SubscriptionTier(code='{self.code}', name='{self.name}'{platform_info})>"
 
     def to_dict(self) -> dict:
         """Convert tier to dictionary (compatible with TIER_LIMITS format)."""
@@ -129,6 +168,8 @@ class SubscriptionTier(Base, TimestampMixin):
             "products_limit": self.products_limit,
             "team_members": self.team_members,
             "order_history_months": self.order_history_months,
+            "cms_pages_limit": self.cms_pages_limit,
+            "cms_custom_pages_limit": self.cms_custom_pages_limit,
             "features": self.features or [],
         }