fix: Use lazy import for create_app in __init__.py

- Change create_app to lazy import to avoid fastapi dependency
- Allows importing from aitbc_chain.cross_chain without triggering fastapi import
- Fixes cross-chain tests with minimal dependencies (pytest pytest-asyncio)

apps/blockchain-node/src/aitbc_chain/__init__.py

@@ -1,5 +1,8 @@
 """AITBC blockchain node package."""
 
-from .app import create_app
+# Lazy import to avoid cascading dependencies
+def create_app():
+    from .app import create_app as _create_app
+    return _create_app()
 
 __all__ = ["create_app"]