Add post-persistence type safety to Post entity

[cameronhotchkies]

The Post entity uses nullable fields (id: Long?, createdAt: Instant?, updatedAt: Instant?) because pre-persistence posts lack these values. After persistence, they are always non-null, but the type system does not reflect this — leading to !! assertions scattered across action/response code.

Options

• Separate Post (pre-persistence) from a PersistedPost (post-persistence) type
• Make repository return types guarantee non-null fields
• Use a sealed type or value class wrapper

Acceptance Criteria

• No !! on post id/timestamps in action or response code
• All tests pass
• No behavioral change