fix: persist edge animation mode (None/Snake/Flow) end-to-end

- canvasStore.onConnect: include animated in edge data object (was silently dropped)
- Backend schemas: normalize animated bool/int to string ('none'/'snake'/'flow') via field_validator
- ORM model: change animated column from Boolean to String
- DB migration: convert existing 0/1 boolean rows to 'none'/'snake' strings

backend/app/schemas/utils.py

@@ -0,0 +1,9 @@
+def normalize_animated(v: object) -> str:
+    """Normalize legacy bool/int animated values to string mode ('none'/'snake'/'flow')."""
+    if v is True or v == 1 or v == '1':
+        return 'snake'
+    if v is False or v == 0 or v == '0' or v is None or v == 'none':
+        return 'none'
+    if v in ('snake', 'flow'):
+        return str(v)
+    return 'none'