Add Posts gRPC Service and WebActions

[cameronhotchkies]

Expose post operations via gRPC and REST/JSON endpoints.

✅ Acceptance Criteria

• POST /api/v1/posts with title and body → returns created post with id and rendered HTML
• GET /api/v1/posts/{id} → returns post with rendered HTML
• PUT /api/v1/posts/{id} → updates post, returns updated content
• DELETE /api/v1/posts/{id} → subsequent GET returns 404
• GET /api/v1/posts → returns posts newest-first
• Creating post without authentication returns 401
• Updating another user's post returns 403

Internal Coding

MP3

• Proto file for PostsService (CreatePost, GetPost, UpdatePost, DeletePost, ListPosts)
• gRPC service implementation
• WebActions for JSON access at /api/v1/posts/*