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

backend/app/db/database.py

@@ -40,6 +40,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN source_handle TEXT")
         with suppress(Exception):
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN target_handle TEXT")
+        with suppress(Exception):
+            await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN animated BOOLEAN NOT NULL DEFAULT 0")
 
 
 async def get_db() -> AsyncGenerator[AsyncSession, None]: