SN 2019qem: decades of extreme mass loss before the explosion of a luminous, long-lived Type IIn supernova

We present the luminous, long-lived Type IIn SN 2019qem and use it to reconstruct how much mass its progenitor lost and when. We combine host-subtracted optical and mid-infrared photometry and 26 optical spectra from -22 to +699 d. We model the bolometric light curve with thin-shell CSM-interaction models in REDBACK, with the shock velocity constrained by the H$α$ line widths, and compare them with an analytic interaction model. SN 2019qem rose in ≈30d to MI = -20.5mag and a bolometric peak of 7.0×10⁴³ erg s⁻¹, and radiated 6.9×10⁵⁰ erg in the optical within 575 d, closely resembling luminous, strongly interacting SNe IIn such as SN 2010jl and SN 2015da. A Heii/Niii flash feature at -22 d and unresolved narrow Balmer lines reveal dense, slow CSM. From +78 to +445 d the H$α$ wings are shaped by electron scattering, and the line stays blueshifted by 400-700 km s⁻¹, which points to an optically thick, asymmetric interaction region rather than to newly formed dust. In the preferred model, ≈7 M_ of ejecta run into ≈9 M_ of gas ejected ≈2 yr before explosion and then into a wind of ≈0.6 M_ yr⁻¹ (for vw = 100 km s⁻¹), sweeping up ≈32 M_ by +544 d. The mid-infrared excess brightens for two years while the optical fades, tracing dust heated by the ongoing interaction. The host is a low-mass, star-forming galaxy, log(M_*/M₎ = 9.19, with a sub-solar metallicity at the explosion site. In its final decades the progenitor of SN 2019qem lost mass far faster than any steady stellar wind, ending with an eruption of ≈9 M_ two years before core collapse, consistent with luminous-blue-variable-like eruptive mass loss. Taken at face value, the mass budget implies an initial progenitor mass of 40 M_.