fix: don't delete transfer records on empty wallet-rpc response

If wallet-rpc returns an empty response (transient network issue,
daemon temporarily unavailable), seen_txids is empty and ALL stored
InvoiceTransfer records for that invoice are deleted. When wallet-rpc
recovers, transfers are re-created with new DB IDs.

This guard only prunes transfers when we actually got data back.

api/app/reconciler.py

@@ -271,10 +271,13 @@ def _sync_invoice_transfers(
             stored.timestamp = transfer.timestamp
             stored.address = transfer.address
             changed = True
-    for stored in existing:
-        if stored.txid not in seen_txids:
-            db.delete(stored)
-            changed = True
+    # Only prune transfers if we actually got data back from wallet-rpc.
+    # An empty response (transient RPC glitch) should not wipe confirmed records.
+    if transfers:
+        for stored in existing:
+            if stored.txid not in seen_txids:
+                db.delete(stored)
+                changed = True
     return changed