feat(api): add V2 JSON:API relationship endpoints for comment resources

[wescopeland]

This PR adds V2 API relationship endpoints for Comment resources, covering the V1 API_GetComments use cases for games, achievements, and user walls.

The resources are accessible through relationship routes:

• GET /games/{game}/comments
• GET /achievements/{achievement}/comments
• GET /users/{user}/wall-comments

Things worth noting:

1. Comments are accessed exclusively via relationship routes. There is no standalone comments index.
2. Results are default sorted by submittedAt, matching V1's ascending default.
3. Default attributes include author display data (authorDisplayName, authorAvatarUrl, authorId), isAutomated, and permalink, so clients can render useful comment lists without requiring ?include=author.
4. The full author relationship can still be included via ?include=author.
5. Deleted comments and comments from banned/deleted authors are excluded.
6. Disabled or banned user walls return 404s for both related-resource and relationship-linkage routes.

How to test this PR

# Get paginated comments for a game (50 per page)
curl --request GET \
  --url http://localhost:64000/api/v2/games/1/comments \
  --header 'X-API-Key: {redacted}'

# Get paginated comments for an achievement
curl --request GET \
  --url http://localhost:64000/api/v2/achievements/9/comments \
  --header 'X-API-Key: {redacted}'

# Get paginated wall comments for a user
curl --request GET \
  --url http://localhost:64000/api/v2/users/Scott/wall-comments \
  --header 'X-API-Key: {redacted}'

# Get comments with full author resources included
curl --request GET \
  --url 'http://localhost:64000/api/v2/games/1/comments?include=author&fields[users]=displayName,avatarUrl' \
  --header 'X-API-Key: {redacted}'

# Sort by newest comments first
curl --request GET \
  --url 'http://localhost:64000/api/v2/games/1/comments?sort=-submittedAt' \
  --header 'X-API-Key: {redacted}'

# Use custom pagination
curl --request GET \
  --url 'http://localhost:64000/api/v2/users/Scott/wall-comments?page[number]=2&page[size]=25' \
  --header 'X-API-Key: {redacted}'