refac: atomic merge, lost-update race, RunPurge cost, duplicated walks

internal/infrastructure/useCases/purgePlan.go

@@ -23,6 +23,7 @@ type purgePlan struct {
 	copies      int
 	articles    int
 	fingerprint string
+	entries     *entryMediaIndex
 }
 
 type purgePlanner struct {
@@ -43,7 +44,15 @@ func (p *purgePlanner) plan() (purgePlan, error) {
 		return purgePlan{}, err
 	}
 
-	siteUsage, err := imageUsageCount(p.sites, p.authors, p.entries)
+	var entryIndex *entryMediaIndex
+	siteUsage, err := combinedImageUsage(p.sites, p.authors, func() (map[uint32]int, error) {
+		built, err := p.entries.index()
+		if err != nil {
+			return nil, err
+		}
+		entryIndex = built
+		return built.usageCounts(), nil
+	})
 	if err != nil {
 		return purgePlan{}, err
 	}
@@ -64,7 +73,7 @@ func (p *purgePlanner) plan() (purgePlan, error) {
 	}
 	slices.SortFunc(groups, func(a, b purgeGroup) int { return int(a.kept.ID) - int(b.kept.ID) })
 
-	return purgePlan{groups: groups, copies: copies, articles: len(touched), fingerprint: fingerprintOf(groups)}, nil
+	return purgePlan{groups: groups, copies: copies, articles: len(touched), fingerprint: fingerprintOf(groups), entries: entryIndex}, nil
 }
 
 func groupByHash(images []image.Image) [][]image.Image {