refuse to update spooldb with negative total or filament weights

main.go

@@ -217,6 +217,10 @@ func processImg(img image.Image, name string, auth claude.Auth, dryRun bool) res
 		r.Error = fmt.Sprintf("scale is set to imperial units (%s); switch it to grams", reading.Unit)
 		return r
 	}
+	if reading.Weight < 0 {
+		r.Error = fmt.Sprintf("scale read a negative weight (%g g); refusing to update", reading.Weight)
+		return r
+	}
 	r.Unit = reading.Unit
 	r.Confidence = ptr(reading.Confidence)
 	r.VoteWeights = reading.Weights
@@ -228,6 +232,10 @@ func processImg(img image.Image, name string, auth claude.Auth, dryRun bool) res
 	if info != nil {
 		r.NewWeight.Spool = ptr(info.EmptySpoolGrams)
 		r.NewWeight.Filament = ptr(reading.Weight - info.EmptySpoolGrams)
+		if reading.Weight-info.EmptySpoolGrams < 0 {
+			r.Error = fmt.Sprintf("measured total %g g is below the empty-spool weight %g g (negative filament); refusing to update", reading.Weight, info.EmptySpoolGrams)
+			return r
+		}
 	}
 
 	// Write the measured total back when it differs from the site's total — the