Run classification previews in the background with live progress

The preview endpoint held one HTTP request open while classifying
serially at three-second pacing, so any real range meant minutes of a
grayed-out button and per-row errors were invisible until the loop
ended. Analyse now starts a single background run against its own
snapshot; a progress endpoint reports analysed counts, proposed
changes and errors as they happen, and the page polls it with a
progress bar, pace-based estimate and a Stop button. Navigating away
no longer orphans the run: the page re-attaches to it on return.

A run that has produced no successful proposal and fails three times
in a row with the identical error stops early and reports that error,
so a wrong key or unsupported model surfaces in seconds instead of
repeating across the whole paced range.

Also normalize a null settings.private_names, which crashed the whole
UI on a workspace that had never saved preferences.

internal/server/server.go

@@ -68,6 +68,7 @@ func New(a *app.App, assets fs.FS, publicURL string) (http.Handler, error) {
 		respond(w, v, e)
 	})
 	s.mux.HandleFunc("POST /api/reclassify/preview", s.preview)
+	s.mux.HandleFunc("POST /api/reclassify/progress", s.previewProgress)
 	s.mux.HandleFunc("POST /api/reclassify/apply", s.apply)
 	s.mux.HandleFunc("POST /api/reclassify/cancel", s.cancel)
 	s.mux.HandleFunc("POST /api/taxonomy/propose", s.taxonomyPropose)
@@ -490,7 +491,17 @@ func (s *Server) preview(w http.ResponseWriter, r *http.Request) {
 	if !decode(w, r, &b) {
 		return
 	}
-	v, e := s.app.Preview(r.Context(), b)
+	v, e := s.app.StartPreview(r.Context(), b)
+	respond(w, v, e)
+}
+func (s *Server) previewProgress(w http.ResponseWriter, r *http.Request) {
+	var b struct {
+		ID string `json:"id"`
+	}
+	if !decode(w, r, &b) {
+		return
+	}
+	v, e := s.app.PreviewProgress(b.ID)
 	respond(w, v, e)
 }
 func (s *Server) apply(w http.ResponseWriter, r *http.Request) {