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>

nodes/fakesink/main.cpp

@@ -6,7 +6,7 @@
 
 class FakeSinkNode : public dmf::NodeBase {
     void run() override {
-        const auto flow_info = config().at("flow_id");
+        const auto flow_info = config().at("video_flow_id");
         const auto flow_id   = flow_info.at("id").get<std::string>();
         const int  fps_num   = flow_info.value("fps_num", 25);
         const int  fps_den   = flow_info.value("fps_den", 1);

nodes/testpattern/main.cpp

@@ -10,7 +10,7 @@
 class TestPatternNode : public dmf::NodeBase {
     void run() override {
         // --- video flow ---
-        const auto flow_info = config().at("flow_id");
+        const auto flow_info = config().at("video_flow_id");
         const auto flow_id   = flow_info.at("id").get<std::string>();
         const int  width     = flow_info.value("width",   1920);
         const int  height    = flow_info.value("height",  1080);