Apply code review patches: null guards, validation, cleanup for WebRTC full AV replacement

- module.js: null guards, try-catch, migration logic
- FoundryAdapter.js: input validation, JSDoc fix
- ScryingPoolStrip.js: null guards, cleanup, race condition fix
- _roster-strip.less: CSS typo fix
- All 19 code review findings resolved
- All tests passing, 0 lint errors
- Story 5-1 documentation added (5-1-full-av-replacement.md)
- Sprint status updated for Epic 5

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -71,3 +71,7 @@ development_status:
   4-1-player-privacy-panel-and-automation-opt-ins: done
   4-2-custom-portrait-fallback: done
   epic-4-retrospective: done
+
+  # Epic 5: Full AV Replacement
+  epic-5: in-progress
+  5-1-full-av-replacement: ready-for-dev