refactor: rename video port flow_id → video_flow_id everywhere

All nodes now use video_flow_id/audio_flow_id consistently:
- nodes/testpattern: flow_id → video_flow_id
- nodes/fakesink:    flow_id → video_flow_id
- graph.json:        from_port/to_port flow_id → video_flow_id
- studio-manager:    hardcoded build_graph edges + FlowGraph.hpp comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

graph.json

@@ -5,8 +5,8 @@
   ],
   "edges": [
     {
-      "from": "testpattern", "from_port": "flow_id",
-      "to":   "ndiout",      "to_port":   "flow_id",
+      "from": "testpattern", "from_port": "video_flow_id",
+      "to":   "ndiout",      "to_port":   "video_flow_id",
       "format": { "kind": "video", "width": 1920, "height": 1080, "fps_num": 25, "fps_den": 1 }
     },
     {