feat: share post to other apps

internal/model/social.go

@@ -19,6 +19,12 @@ type Post struct {
 	AuthorName      string  `json:"author_name,omitempty"`
 	AuthorHandle    string  `json:"author_handle,omitempty"`
 	AuthorAvatarURL *string `json:"author_avatar_url,omitempty"`
+
+	// ViewerHasReshared is true when the calling user has reshared this
+	// post. Hydrated by the service layer after the repo returns posts;
+	// always false for anonymous viewers (no viewer = no reshare state).
+	// Drives the post-card "Reshare ↔ Send" swap on the frontend.
+	ViewerHasReshared bool `json:"viewer_has_reshared"`
 }
 
 // PostImage represents one attached image on a post.