Improve merchant creation with automatic keypair generation

Enhance the merchant creation process by automatically generating Nostr keypairs
for users who don't have them, and streamline the API interface.

Changes:
- Add CreateMerchantRequest model to simplify merchant creation API
- Auto-generate Nostr keypairs for users without existing keys
- Update merchant creation endpoint to use user account keypairs
- Improve error handling and validation in merchant creation flow
- Clean up frontend JavaScript for merchant creation

This ensures all merchants have proper Nostr keypairs for marketplace
functionality without requiring manual key management from users.

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

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

models.py

@@ -49,6 +49,10 @@ class MerchantConfig(MerchantProfile):
     restore_in_progress: bool | None = False
 
 
+class CreateMerchantRequest(BaseModel):
+    config: MerchantConfig = MerchantConfig()
+
+
 class PartialMerchant(BaseModel):
     private_key: str
     public_key: str