fix(workspace): add derives/docs per code review, remove unused thiserror from solitaire_sync

solitaire_sync/Cargo.toml

@@ -8,4 +8,3 @@ serde      = { workspace = true }
 serde_json = { workspace = true }
 uuid       = { workspace = true }
 chrono     = { workspace = true }
-thiserror  = { workspace = true }

solitaire_sync/src/lib.rs

@@ -4,14 +4,14 @@ use uuid::Uuid;
 
 /// Payload sent from client to server (and returned after server merge).
 /// Full fields are added in Phase 8 (Sync System).
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct SyncPayload {
     pub user_id: Uuid,
     pub last_modified: DateTime<Utc>,
 }
 
 /// Response returned by the sync server after merging.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct SyncResponse {
     pub server_time: DateTime<Utc>,
 }