feat(posts): add digest field to UpdatePostRequest for idempotent updates

[cameronhotchkies]

Summary

Add a digest field to UpdatePostRequest so the server can reject stale updates (debouncing). The digest is a lightweight hash (CRC32 or MD5) of the post content — no cryptographic security needed.

Acceptance Criteria

• User can submit an update with a digest of the content they based their edit on
• Server rejects the update if the digest doesn't match the current post content (409 Conflict)
• Digest algorithm is simple (CRC32 or similar) — not cryptographically secure
• Digest field is optional for backward compatibility

Context

From PR #31 review feedback on posts.proto.