fix: replace O(n²) duplicate detection with map-based O(1) lookups

Replace linear scan deduplication of snapshot IDs in RemoveAllSnapshots()
and PruneBlobs() with map[string]bool for O(1) lookups.

Previously, each new snapshot ID was checked against the entire collected
slice via a linear scan, resulting in O(n²) overall complexity. Now a
'seen' map provides constant-time membership checks while preserving
insertion order in the slice.

closes #12

internal/vaultik/prune.go

@@ -35,6 +35,7 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 	log.Info("Listing remote snapshots")
 	objectCh := v.Storage.ListStream(v.ctx, "metadata/")
 
+	seen := make(map[string]bool)
 	var snapshotIDs []string
 	for object := range objectCh {
 		if object.Err != nil {
@@ -47,15 +48,8 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 			// Check if this is a directory by looking for trailing slash
 			if strings.HasSuffix(object.Key, "/") || strings.Contains(object.Key, "/manifest.json.zst") {
 				snapshotID := parts[1]
-				// Only add unique snapshot IDs
-				found := false
-				for _, id := range snapshotIDs {
-					if id == snapshotID {
-						found = true
-						break
-					}
-				}
-				if !found {
+				if !seen[snapshotID] {
+					seen[snapshotID] = true
 					snapshotIDs = append(snapshotIDs, snapshotID)
 				}
 			}

internal/vaultik/snapshot.go

@@ -913,6 +913,7 @@ func (v *Vaultik) RemoveAllSnapshots(opts *RemoveOptions) (*RemoveResult, error)
 	log.Info("Listing all snapshots")
 	objectCh := v.Storage.ListStream(v.ctx, "metadata/")
 
+	seen := make(map[string]bool)
 	var snapshotIDs []string
 	for object := range objectCh {
 		if object.Err != nil {
@@ -927,14 +928,8 @@ func (v *Vaultik) RemoveAllSnapshots(opts *RemoveOptions) (*RemoveResult, error)
 			}
 			if strings.HasSuffix(object.Key, "/") || strings.Contains(object.Key, "/manifest.json.zst") {
 				sid := parts[1]
-				found := false
-				for _, id := range snapshotIDs {
-					if id == sid {
-						found = true
-						break
-					}
-				}
-				if !found {
+				if !seen[sid] {
+					seen[sid] = true
 					snapshotIDs = append(snapshotIDs, sid)
 				}
 			}