fix: add vendor page creation endpoint and update JS logic

- Add POST /admin/content-pages/vendor endpoint for vendor overrides
- Update JS to use /vendor or /platform endpoint based on vendor selection
- Platform endpoint forces vendor_id=None, vendor endpoint requires vendor_id

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/admin/content_pages.py

@@ -161,6 +161,52 @@ def create_platform_page(
     return page.to_dict()
 
 
+# ============================================================================
+# VENDOR PAGES
+# ============================================================================
+
+
+@router.post("/vendor", response_model=ContentPageResponse, status_code=201)
+def create_vendor_page(
+    page_data: ContentPageCreate,
+    current_user: User = Depends(get_current_admin_api),
+    db: Session = Depends(get_db),
+):
+    """
+    Create a vendor-specific content page override.
+
+    Vendor pages override platform defaults for a specific vendor.
+    """
+    if not page_data.vendor_id:
+        from fastapi import HTTPException
+
+        raise HTTPException(
+            status_code=400,
+            detail="vendor_id is required for vendor pages. Use /platform for platform defaults.",
+        )
+
+    page = content_page_service.create_page(
+        db,
+        slug=page_data.slug,
+        title=page_data.title,
+        content=page_data.content,
+        vendor_id=page_data.vendor_id,
+        content_format=page_data.content_format,
+        template=page_data.template,
+        meta_description=page_data.meta_description,
+        meta_keywords=page_data.meta_keywords,
+        is_published=page_data.is_published,
+        show_in_footer=page_data.show_in_footer,
+        show_in_header=page_data.show_in_header,
+        show_in_legal=page_data.show_in_legal,
+        display_order=page_data.display_order,
+        created_by=current_user.id,
+    )
+    db.commit()
+
+    return page.to_dict()
+
+
 # ============================================================================
 # ALL CONTENT PAGES (Platform + Vendors)
 # ============================================================================