[Fix] Add related field to notifications_response swagger spec (#801)

## Summary
- [PR #799](#799) added a
`related` block (`{ users, tracks, playlists }`) to the
`/v1/notifications` response in Go but didn't update the swagger spec to
match.
- Without this, generated SDK clients (e.g. `@audius/sdk`) don't see the
field and consumers can't read it in a type-safe way.
- One-line addition — `$ref`s the existing `related` schema (already
defined at line 13146 for other endpoints).

## Test plan
- [x] Spec lints / parses (single property addition under existing
schema)
- [ ] Regenerating SDK clients picks up `related?: Related` on
`NotificationsResponse` (verified locally against the `@audius/sdk`
typescript-fetch generator)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dylanjeffers

api/swagger/swagger-v1.yaml

@@ -15212,6 +15212,8 @@ components:
           $ref: "#/components/schemas/version_metadata"
         data:
           $ref: "#/components/schemas/notifications"
+        related:
+          $ref: "#/components/schemas/related"
     connected_wallets_response:
       type: object
       properties: