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>

models.py

@@ -43,6 +43,8 @@ class MerchantConfig(MerchantProfile):
     # TODO: switched to True for AIO demo; determine if we leave this as True
     active: bool = True
     restore_in_progress: Optional[bool] = False
+    # Set at runtime (not persisted) when account keypair != merchant keypair
+    key_mismatch: Optional[bool] = False
 
 
 class CreateMerchantRequest(BaseModel):