Stage 17 round 6 (#10 backend): enforce chat only on your turn

PostMessage now rejects a chat sent on a finished game or when it is not the sender's
turn (ErrChatNotYourTurn -> 409 chat_not_your_turn), matching the UI where the message
field is hidden off-turn and only the nudge shows. Existing chat tests post on the
to-move seat and are unaffected; adds an off-turn-rejection integration test + the dto
mapping case + the UI error message.

backend/internal/server/handlers.go

@@ -207,6 +207,8 @@ func statusForError(err error) (int, string) {
 		// A too-frequent nudge is a distinct, non-content rejection — the UI must say
 		// "don't rush the player so often", not the chat content-rejection message.
 		return http.StatusConflict, "nudge_too_soon"
+	case errors.Is(err, social.ErrChatNotYourTurn):
+		return http.StatusConflict, "chat_not_your_turn"
 	case errors.Is(err, social.ErrSelfRelation):
 		return http.StatusBadRequest, "self_relation"
 	case errors.Is(err, social.ErrRequestExists):