feat: Add inpainting (局部重绘) feature for timeline editor

- Add canvas-based mask drawing tools (brush, eraser, rect, lasso)
- Add undo/redo history support for mask editing
- Integrate inpainting UI into preview player
- Add backend API endpoint for inpainting requests
- Add MediaService.inpaint method with ComfyUI workflow support
- Add Flux inpainting workflows for selfhost and RunningHub

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

workflows/runninghub/image_inpaint.json

@@ -0,0 +1,5 @@
+{
+  "source": "runninghub",
+  "workflow_id": "YOUR_INPAINT_WORKFLOW_ID",
+  "description": "Flux Inpainting workflow on RunningHub - Replace workflow_id with your actual RunningHub workflow ID"
+}