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>

static/js/index.js

@@ -21,26 +21,18 @@ window.app = Vue.createApp({
   },
   methods: {
     generateKeys: async function () {
-      const privateKey = nostr.generatePrivateKey()
-      await this.createMerchant(privateKey)
+      // No longer need to generate keys here - the backend will use user's existing keypairs
+      await this.createMerchant()
     },
     importKeys: async function () {
       this.importKeyDialog.show = false
-      let privateKey = this.importKeyDialog.data.privateKey
-      if (!privateKey) {
-        return
-      }
-      try {
-        if (privateKey.toLowerCase().startsWith('nsec')) {
-          privateKey = nostr.nip19.decode(privateKey).data
-        }
-      } catch (error) {
-        this.$q.notify({
-          type: 'negative',
-          message: `${error}`
-        })
-      }
-      await this.createMerchant(privateKey)
+      // Import keys functionality removed since we use user's native keypairs
+      // Show a message that this is no longer needed
+      this.$q.notify({
+        type: 'info',
+        message: 'Merchants now use your account Nostr keys automatically. Key import is no longer needed.',
+        timeout: 3000
+      })
     },
     showImportKeysDialog: async function () {
       this.importKeyDialog.show = true
@@ -94,12 +86,9 @@ window.app = Vue.createApp({
       this.activeChatCustomer = ''
       this.showKeys = false
     },
-    createMerchant: async function (privateKey) {
+    createMerchant: async function () {
       try {
-        const pubkey = nostr.getPublicKey(privateKey)
         const payload = {
-          private_key: privateKey,
-          public_key: pubkey,
           config: {}
         }
         const {data} = await LNbits.api.request(