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-1.2.json

@@ -0,0 +1,23 @@
+{
+  "assignee": "igor.loskutoff@gmail.com",
+  "claim_note": "",
+  "claimed_at": "2026-01-12T23:11:46.263763Z",
+  "created_at": "2026-01-12T22:41:17.501928Z",
+  "depends_on": [],
+  "epic": "fn-1",
+  "evidence": {
+    "commits": [
+      "dbb619e7fcf50634c6bc7b7a355183de2243131b"
+    ],
+    "prs": [],
+    "tests": [
+      "pytest tests/test_transcript_formats.py::test_topics_to_webvtt_named"
+    ]
+  },
+  "id": "fn-1.2",
+  "priority": null,
+  "spec_path": ".flow/tasks/fn-1.2.md",
+  "status": "done",
+  "title": "WebVTT context generation",
+  "updated_at": "2026-01-12T23:21:46.532277Z"
+}