refactor: single refusal shape with openapi contract test

crates/api-types/src/common.rs

@@ -21,3 +21,12 @@ pub type ActivityFeedResponse = PaginatedResponse<crate::diary::FeedEntryDto>;
 pub type WatchlistResponse = PaginatedResponse<crate::watchlist::WatchlistEntryDto>;
 pub type PaginatedMovieHits = PaginatedResponse<crate::search::MovieSearchHitDto>;
 pub type PaginatedPersonHits = PaginatedResponse<crate::search::PersonSearchHitDto>;
+
+/// The one body every JSON API refusal carries.
+#[derive(Debug, Clone, Serialize, Deserialize, utoipa::ToSchema)]
+pub struct ErrorResponse {
+    /// Human-readable message.
+    pub error: String,
+    /// Stable machine-readable code, e.g. `NOT_FOUND`.
+    pub code: String,
+}