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/vendor_platform.py

@@ -0,0 +1,189 @@
+# models/database/vendor_platform.py
+"""
+VendorPlatform junction table for many-to-many relationship between Vendor and Platform.
+
+A vendor CAN belong to multiple platforms (e.g., both OMS and Loyalty Program).
+Each membership can have:
+- Platform-specific subscription tier
+- Custom subdomain for that platform
+- Platform-specific settings
+- Active/inactive status
+"""
+
+from datetime import UTC, datetime
+
+from sqlalchemy import (
+    JSON,
+    Boolean,
+    Column,
+    DateTime,
+    ForeignKey,
+    Index,
+    Integer,
+    String,
+    UniqueConstraint,
+)
+from sqlalchemy.orm import relationship
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class VendorPlatform(Base, TimestampMixin):
+    """
+    Junction table linking vendors to platforms.
+
+    Allows a vendor to:
+    - Subscribe to multiple platforms (OMS + Loyalty)
+    - Have different tiers per platform
+    - Have platform-specific subdomains
+    - Store platform-specific settings
+
+    Example:
+    - Vendor "WizaMart" is on OMS platform (Professional tier)
+    - Vendor "WizaMart" is also on Loyalty platform (Basic tier)
+    """
+
+    __tablename__ = "vendor_platforms"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    # ========================================================================
+    # Foreign Keys
+    # ========================================================================
+
+    vendor_id = Column(
+        Integer,
+        ForeignKey("vendors.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Reference to the vendor",
+    )
+
+    platform_id = Column(
+        Integer,
+        ForeignKey("platforms.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Reference to the platform",
+    )
+
+    tier_id = Column(
+        Integer,
+        ForeignKey("subscription_tiers.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="Platform-specific subscription tier",
+    )
+
+    # ========================================================================
+    # Membership Status
+    # ========================================================================
+
+    is_active = Column(
+        Boolean,
+        default=True,
+        nullable=False,
+        comment="Whether the vendor is active on this platform",
+    )
+
+    is_primary = Column(
+        Boolean,
+        default=False,
+        nullable=False,
+        comment="Whether this is the vendor's primary platform",
+    )
+
+    # ========================================================================
+    # Platform-Specific Configuration
+    # ========================================================================
+
+    custom_subdomain = Column(
+        String(100),
+        nullable=True,
+        comment="Platform-specific subdomain (if different from main subdomain)",
+    )
+
+    settings = Column(
+        JSON,
+        nullable=True,
+        default=dict,
+        comment="Platform-specific vendor settings",
+    )
+
+    # ========================================================================
+    # Timestamps
+    # ========================================================================
+
+    joined_at = Column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+        comment="When the vendor joined this platform",
+    )
+
+    # ========================================================================
+    # Relationships
+    # ========================================================================
+
+    vendor = relationship(
+        "Vendor",
+        back_populates="vendor_platforms",
+    )
+
+    platform = relationship(
+        "Platform",
+        back_populates="vendor_platforms",
+    )
+
+    tier = relationship(
+        "SubscriptionTier",
+        foreign_keys=[tier_id],
+    )
+
+    # ========================================================================
+    # Constraints & Indexes
+    # ========================================================================
+
+    __table_args__ = (
+        # Each vendor can only be on a platform once
+        UniqueConstraint(
+            "vendor_id",
+            "platform_id",
+            name="uq_vendor_platform",
+        ),
+        # Performance indexes
+        Index(
+            "idx_vendor_platform_active",
+            "vendor_id",
+            "platform_id",
+            "is_active",
+        ),
+        Index(
+            "idx_vendor_platform_primary",
+            "vendor_id",
+            "is_primary",
+        ),
+    )
+
+    # ========================================================================
+    # Properties
+    # ========================================================================
+
+    @property
+    def tier_code(self) -> str | None:
+        """Get the tier code for this platform membership."""
+        return self.tier.code if self.tier else None
+
+    @property
+    def tier_name(self) -> str | None:
+        """Get the tier name for this platform membership."""
+        return self.tier.name if self.tier else None
+
+    def __repr__(self) -> str:
+        return (
+            f"<VendorPlatform("
+            f"vendor_id={self.vendor_id}, "
+            f"platform_id={self.platform_id}, "
+            f"is_active={self.is_active})>"
+        )