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/snapshot.go

@@ -988,6 +988,7 @@ func (v *Vaultik) listAllRemoteSnapshotIDs() ([]string, 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 {
@@ -1002,14 +1003,8 @@ func (v *Vaultik) listAllRemoteSnapshotIDs() ([]string, 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)
 				}
 			}