Add default Neo4j credentials to docker-compose

Set sensible defaults (neo4j/password) to prevent NEO4J_AUTH error
when .env file is not present.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -22,8 +22,8 @@ services:
     environment:
       - OPENAI_API_KEY=${OPENAI_API_KEY}
       - NEO4J_URI=bolt://neo4j:${NEO4J_PORT:-7687}
-      - NEO4J_USER=${NEO4J_USER}
-      - NEO4J_PASSWORD=${NEO4J_PASSWORD}
+      - NEO4J_USER=${NEO4J_USER:-neo4j}
+      - NEO4J_PASSWORD=${NEO4J_PASSWORD:-password}
       - PORT=8000
       - db_backend=neo4j
   neo4j:
@@ -45,7 +45,7 @@ services:
     volumes:
       - neo4j_data:/data
     environment:
-      - NEO4J_AUTH=${NEO4J_USER}/${NEO4J_PASSWORD}
+      - NEO4J_AUTH=${NEO4J_USER:-neo4j}/${NEO4J_PASSWORD:-password}
 
   falkordb:
     image: falkordb/falkordb:latest