Gap Fill Epw

Repair an uploaded EPW: identify missing hours / suspect runs from the QC report and rebuild the affected columns from reanalysis + nearest-station observations at the EPW's own (lat, lon, year).

Internally this is a wrapper around the same MERRA-2 + Meteostat pipeline used by /api/generate/individual, run only for the gaps. The user-facing label uses "MOSMIX" because that's the brand the gap-fill workflow is associated with on the site, even though MOSMIX itself is forecast-only.