feat: add platform assignment to user and vendor creation forms

User create page:
- When role=admin, show super admin toggle
- If not super admin, show platform multi-select
- Admin users created via /api/v1/admin/admin-users endpoint
- Vendor users created via existing /admin/users endpoint

Vendor create page:
- Add platform selection section
- Vendors can be assigned to multiple platforms on creation
- Update VendorCreate schema to accept platform_ids
- Update AdminService.create_vendor() to create VendorPlatform records

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

app/services/admin_service.py

@@ -34,6 +34,7 @@ from app.exceptions.auth import UserAlreadyExistsException
 from middleware.auth import AuthManager
 from models.database.company import Company
 from models.database.marketplace_import_job import MarketplaceImportJob
+from models.database.platform import Platform
 from models.database.user import User
 from models.database.vendor import Role, Vendor
 from models.schema.marketplace_import_job import MarketplaceImportJobResponse
@@ -419,6 +420,24 @@ class AdminService:
             # Create default roles for vendor
             self._create_default_roles(db, vendor.id)
 
+            # Assign vendor to platforms if provided
+            if vendor_data.platform_ids:
+                from models.database.vendor_platform import VendorPlatform
+
+                for platform_id in vendor_data.platform_ids:
+                    # Verify platform exists
+                    platform = db.query(Platform).filter(Platform.id == platform_id).first()
+                    if platform:
+                        vendor_platform = VendorPlatform(
+                            vendor_id=vendor.id,
+                            platform_id=platform_id,
+                            is_active=True,
+                        )
+                        db.add(vendor_platform)
+                        logger.debug(
+                            f"Assigned vendor {vendor.vendor_code} to platform {platform.code}"
+                        )
+
             db.flush()
             db.refresh(vendor)