fix(docker): update regtest network/volume names

Regtest stack renamed from lnbits_ prefix to regtest_ prefix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.regtest.yml

@@ -142,11 +142,9 @@ volumes:
   # Mount the bitcoin-data volume from the regtest environment
   bitcoin-data:
     external: true
-    name: lnbits_bitcoin-data
+    name: regtest_bitcoin-data
 
 networks:
   regtest:
-    # The regtest environment uses 'lnbits_default' as its Docker network
-    # (named after the original LNbits regtest project)
-    name: lnbits_default
+    name: regtest_default
     external: true