[Follow-up #255] Replace String.to_existing_atom with explicit whitelist in LikeNormalizer

[podclaude]

Context

Follow-up from PR #255: feat: add like/unlike system

Original Finding

String.to_existing_atom in LikeNormalizer works because :liked_at, :unliked_at, :rss_source_feed are defined in struct modules. However, if a field is renamed in a future refactor, this will raise ArgumentError on checkpoint replay with no helpful error message.

Acceptance Criteria

• Replace String.to_existing_atom with explicit whitelist map in LikeNormalizer
• Add test for unknown key handling