feat: add in-product survey system (#2008)

* feat: add in-product survey system

- SurveyManager: event-based trigger, Space API communication
- Trigger on first successful non-WebSocket response
- Backend API: /api/v1/survey/{pending,respond,dismiss}
- Frontend: floating survey widget with progressive questions
- Flat radio/checkbox style (not dropdown Select)

* fix: persist triggered survey events to disk across restarts

Store triggered events in data/survey_triggered_events.json so that
restarting the process doesn't re-query Space for already-triggered events.

* fix: use metadata table for survey event persistence instead of file

Store triggered events in the existing metadata KV table
(key='survey_triggered_events') instead of a standalone JSON file.

* fix: ruff format and prettier fixes

src/langbot/pkg/survey/__init__.py

@@ -0,0 +1 @@
+"""Survey module for in-product surveys triggered by events."""