Rename persisted unread storage to read semantics

[johan456789]

Summary

Finish the read-state naming cleanup in the persistence layer.

Scope

• Rename persisted Room model fields from isUnread to isRead
• Decide whether to keep mapping to the legacy database column or rename the actual SQLite column
• Add and verify any required Room migration
• Update exported Room schemas if the storage schema changes

Goal

Remove the remaining storage-layer naming debt after the app-facing and repository-layer cleanup in issue #22.