feat: add WebVTT context generation to chat WebSocket endpoint

- Import topics_to_webvtt_named and recordings controller
- Add _get_is_multitrack helper function
- Generate WebVTT context on WebSocket connection
- Add get_context message type to retrieve WebVTT
- Maintain backward compatibility with echo for other messages
- Add test fixture and test for WebVTT context generation

Implements task fn-1.2: WebVTT context generation for transcript chat

.flow/tasks/fn-1.7.md

@@ -0,0 +1,15 @@
+# fn-1.7 Integrate into transcript page
+
+## Description
+TBD
+
+## Acceptance
+- [ ] TBD
+
+## Done summary
+TBD
+
+## Evidence
+- Commits:
+- Tests:
+- PRs: