feat: add edge flow animation (dot traveling source→target)

- Add animated toggle per edge in EdgeModal (cyan switch, "Flow Animation")
- SVG-native <animate> element for reliable cross-browser dot animation
- Persist animated field: backend model, schemas (EdgeBase/EdgeUpdate/EdgeSave), DB migration
- Include animated in App.tsx edgesToSave serialization so it survives save/reload
- Add animated: bool to EdgeData TypeScript type

frontend/src/types/index.ts

@@ -81,6 +81,7 @@ export interface EdgeData extends Record<string, unknown> {
   speed?: string
   custom_color?: string
   path_style?: EdgePathStyle
+  animated?: boolean
 }
 
 export const NODE_TYPE_LABELS: Record<NodeType, string> = {