[APIView] Epic: Version-Scoped Comments

## Overview

Version-scoped comments for APIView. Eliminates cross-version comment bleed by scoping the code panel to the active version's comments, with an opt-in toggle to view comments from other versions.

## Work Items

- [ ] #14998 — WI-7: Comment FK & Backfill
- [ ] #14999 — WI-8: Comment Query Switch & Frontend Update
- [ ] #15000 — WI-9: Cross-Version Comment Toggle

## Dependency Chain

```
WI-3 → WI-7 → WI-8 → WI-9
```

⚠️ **Stable-state gate:** WI-7 backfill must be verified complete before flipping the WI-8 feature flag. Comments without `APIVersionId` would be invisible if WI-8 activates prematurely.