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/index.css

@@ -115,3 +115,13 @@
 .font-mono {
     font-family: 'JetBrains Mono', monospace;
 }
+
+/* Edge flow animation — dot traveling from source to target */
+@keyframes flow-dot {
+    from { stroke-dashoffset: 0; }
+    to   { stroke-dashoffset: -10000; }
+}
+.edge-flow-dot {
+    animation: flow-dot 2.5s linear infinite;
+    pointer-events: none;
+}