Add keypair rotation detection and migration feature

When a user rotates their Nostr keypair in account settings, the
merchant still holds the old key. This adds:

- key_mismatch flag on MerchantConfig (runtime, not persisted) -
  detected on each GET /api/v1/merchant by comparing account vs
  merchant pubkey
- POST /api/v1/merchant/{id}/migrate-keys endpoint that updates
  the merchant keys, republishes all stalls/products under the new
  identity, and resubscribes
- Warning banner in the UI with a "Migrate Keys" button and
  confirmation dialog
- update_merchant_keys() crud function

Orders and DM history are preserved since they reference customer
pubkeys. Old stall/product events on relays become orphaned.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/js/index.js

@@ -36,6 +36,30 @@ window.app = Vue.createApp({
     }
   },
   methods: {
+    migrateKeys: async function () {
+      LNbits.utils
+        .confirmDialog(
+          'This will update your merchant to use your current account Nostr keypair ' +
+            'and republish all stalls and products under the new identity. ' +
+            'Existing orders and messages are preserved. Continue?'
+        )
+        .onOk(async () => {
+          try {
+            const {data} = await LNbits.api.request(
+              'POST',
+              `/nostrmarket/api/v1/merchant/${this.merchant.id}/migrate-keys`,
+              this.g.user.wallets[0].adminkey
+            )
+            this.merchant = data
+            this.$q.notify({
+              type: 'positive',
+              message: 'Merchant keys migrated and stalls republished'
+            })
+          } catch (error) {
+            LNbits.utils.notifyApiError(error)
+          }
+        })
+    },
     toggleShowKeys: function () {
       this.showKeys = !this.showKeys
     },