refactor(api): remove redundant content fields from view models
Align PostView and CommentView with atProto patterns by accessing content
through the Record object instead of redundant top-level fields. This matches
the Bluesky approach where the record contains the authoritative content.
Changes:
- Remove content field from CommentView (access via Record.Content)
- Remove title/text fields from PostView (access via Record)
- Update lexicons to remove redundant field definitions
- Update comment_service, feed_repo_base, post_repo to stop setting removed fields
- Add test for deleted comments with nil Record
- Add helper functions in integration tests to extract content from Record
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>