feat: complete multi-platform CMS phases 2-5

Phase 2 - OMS Migration & Integration:
- Fix platform_pages.py to use get_platform_page for marketing pages
- Fix shop_pages.py to pass platform_id to content page service calls

Phase 3 - Admin Interface:
- Add platform management API (app/api/v1/admin/platforms.py)
- Add platforms admin page with stats cards
- Add Platforms menu item to admin sidebar
- Update content pages admin with platform filter and four-tab tier system

Phase 4 - Documentation:
- Add comprehensive architecture docs (docs/architecture/multi-platform-cms.md)
- Update implementation plan with completion status

Phase 5 - Vendor Dashboard:
- Add CMS usage API endpoint with tier limits
- Add usage progress bar to vendor content pages
- Add platform-default/{slug} API for preview
- Add View Default button and modal in page editor

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/vendor/content_pages.py

@@ -97,6 +97,20 @@ class ContentPageResponse(BaseModel):
     updated_by: int | None
 
 
+class CMSUsageResponse(BaseModel):
+    """Schema for CMS usage statistics."""
+
+    total_pages: int
+    custom_pages: int
+    override_pages: int
+    pages_limit: int | None
+    custom_pages_limit: int | None
+    can_create_page: bool
+    can_create_custom: bool
+    usage_percent: float
+    custom_usage_percent: float
+
+
 # ============================================================================
 # VENDOR CONTENT PAGES
 # ============================================================================
@@ -241,3 +255,103 @@ def delete_vendor_page(
     # Delete with ownership check in service layer
     content_page_service.delete_vendor_page(db, page_id, current_user.token_vendor_id)
     db.commit()
+
+
+# ============================================================================
+# CMS USAGE & LIMITS
+# ============================================================================
+
+
+@router.get("/usage", response_model=CMSUsageResponse)
+def get_cms_usage(
+    current_user: User = Depends(get_current_vendor_api),
+    db: Session = Depends(get_db),
+):
+    """
+    Get CMS usage statistics for the vendor.
+
+    Returns page counts and limits based on subscription tier.
+    """
+    from models.database.vendor import Vendor
+
+    vendor = db.query(Vendor).filter(Vendor.id == current_user.token_vendor_id).first()
+    if not vendor:
+        return CMSUsageResponse(
+            total_pages=0,
+            custom_pages=0,
+            override_pages=0,
+            pages_limit=3,
+            custom_pages_limit=0,
+            can_create_page=False,
+            can_create_custom=False,
+            usage_percent=0,
+            custom_usage_percent=0,
+        )
+
+    # Get vendor's pages
+    vendor_pages = content_page_service.list_all_vendor_pages(
+        db, vendor_id=current_user.token_vendor_id, include_unpublished=True
+    )
+
+    total_pages = len(vendor_pages)
+    override_pages = sum(1 for p in vendor_pages if p.is_vendor_override)
+    custom_pages = total_pages - override_pages
+
+    # Get limits from subscription tier
+    pages_limit = None
+    custom_pages_limit = None
+    if vendor.subscription and vendor.subscription.tier:
+        pages_limit = vendor.subscription.tier.cms_pages_limit
+        custom_pages_limit = vendor.subscription.tier.cms_custom_pages_limit
+
+    # Calculate can_create flags
+    can_create_page = pages_limit is None or total_pages < pages_limit
+    can_create_custom = custom_pages_limit is None or custom_pages < custom_pages_limit
+
+    # Calculate usage percentages
+    usage_percent = 0 if pages_limit is None else min(100, (total_pages / pages_limit) * 100) if pages_limit > 0 else 100
+    custom_usage_percent = 0 if custom_pages_limit is None else min(100, (custom_pages / custom_pages_limit) * 100) if custom_pages_limit > 0 else 100
+
+    return CMSUsageResponse(
+        total_pages=total_pages,
+        custom_pages=custom_pages,
+        override_pages=override_pages,
+        pages_limit=pages_limit,
+        custom_pages_limit=custom_pages_limit,
+        can_create_page=can_create_page,
+        can_create_custom=can_create_custom,
+        usage_percent=usage_percent,
+        custom_usage_percent=custom_usage_percent,
+    )
+
+
+@router.get("/platform-default/{slug}", response_model=ContentPageResponse)
+def get_platform_default(
+    slug: str,
+    current_user: User = Depends(get_current_vendor_api),
+    db: Session = Depends(get_db),
+):
+    """
+    Get the platform default content for a slug.
+
+    Useful for vendors to view the original before/after overriding.
+    """
+    from models.database.vendor import Vendor
+
+    # Get vendor's platform
+    vendor = db.query(Vendor).filter(Vendor.id == current_user.token_vendor_id).first()
+    platform_id = 1  # Default to OMS
+
+    if vendor and vendor.platforms:
+        platform_id = vendor.platforms[0].id
+
+    # Get platform default (vendor_id=None)
+    page = content_page_service.get_vendor_default_page(
+        db, platform_id=platform_id, slug=slug, include_unpublished=True
+    )
+
+    if not page:
+        from app.exceptions import NotFoundException
+        raise NotFoundException(f"No platform default found for slug: {slug}")
+
+    return page.to_dict()