Return every error as {"error": "..."}

Echo errors, middleware and handlers used three different formats (message object, error object, bare string). One error handler now writes all of them the same way; status codes are unchanged.

internal/server/middleware/rate_limit.go

@@ -23,9 +23,7 @@ func RateLimit(limit int, window time.Duration) echo.MiddlewareFunc {
 		logging.GetLogger().Warn("Rate limit exceeded",
 			zap.String("ip", identifier),
 			zap.String("path", c.Request().URL.Path))
-		return c.JSON(http.StatusTooManyRequests, map[string]string{
-			"error": "Rate limit exceeded. Please try again later.",
-		})
+		return echo.NewHTTPError(http.StatusTooManyRequests, "Rate limit exceeded. Please try again later.")
 	}
 	return echoMiddleware.RateLimiterWithConfig(config)
 }