feat normalize publishedAt across response models

[Priveetee]

Summary

• add canonical publishedAt fields across shared response models used by search/trending/channel/streams/feed/comments/notifications/event payloads
• normalize extractor timestamps with nullable semantics (null when unavailable) via a dedicated mapper instead of sentinel values
• add serialization and mapper tests to lock the canonical date contract

Validation

• local: ./gradlew test
• CI/Coverage/Docker on dev are green for head 79dcf29