consensus: add error handling for state root computation and database initialization

- Wrap _compute_state_root in try/except to handle failures during genesis block creation
- Return None and log warning when state root computation fails (e.g., when accounts don't exist yet)
- Add error handling in init_db to ignore "already exists" errors when creating tables
- Improve robustness of database initialization for concurrent or repeated startup scenarios

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

@@ -80,7 +80,12 @@ _engine_internal = _engine
 def init_db() -> None:
     """Initialize database with file-based encryption"""
     settings.db_path.parent.mkdir(parents=True, exist_ok=True)
-    SQLModel.metadata.create_all(_engine)
+    try:
+        SQLModel.metadata.create_all(_engine)
+    except Exception as e:
+        # If tables already exist, that's okay
+        if "already exists" not in str(e):
+            raise
     # Set restrictive file permissions on database file
     if settings.db_path.exists():
         os.chmod(settings.db_path, stat.S_IRUSR | stat.S_IWUSR)  # Read/write for owner only