fix: remove destructive sync from ListSnapshots

ListSnapshots() silently deleted local snapshot records not found in
remote storage. A list/read operation should not have destructive side
effects.

Changes:
- Remove the inline deletion loop from ListSnapshots() that deleted
  local snapshots not present in remote storage
- Update syncWithRemote() to use deleteSnapshotFromLocalDB() for
  proper cascade cleanup (deleting snapshot_files, snapshot_blobs,
  and snapshot_uploads before the snapshot record itself)

The sync behavior remains available via syncWithRemote(), which is
called explicitly by PurgeSnapshots().

closes #15

internal/database/schema.sql

@@ -103,7 +103,7 @@ CREATE TABLE IF NOT EXISTS snapshot_files (
     file_id TEXT NOT NULL,
     PRIMARY KEY (snapshot_id, file_id),
     FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE,
-    FOREIGN KEY (file_id) REFERENCES files(id) ON DELETE CASCADE
+    FOREIGN KEY (file_id) REFERENCES files(id)
 );
 
 -- Index for efficient file lookups (used in orphan detection)
@@ -116,7 +116,7 @@ CREATE TABLE IF NOT EXISTS snapshot_blobs (
     blob_hash TEXT NOT NULL,
     PRIMARY KEY (snapshot_id, blob_id),
     FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE,
-    FOREIGN KEY (blob_id) REFERENCES blobs(id) ON DELETE CASCADE
+    FOREIGN KEY (blob_id) REFERENCES blobs(id)
 );
 
 -- Index for efficient blob lookups (used in orphan detection)
@@ -130,7 +130,7 @@ CREATE TABLE IF NOT EXISTS uploads (
     size INTEGER NOT NULL,
     duration_ms INTEGER NOT NULL,
     FOREIGN KEY (blob_hash) REFERENCES blobs(blob_hash),
-    FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE
+    FOREIGN KEY (snapshot_id) REFERENCES snapshots(id)
 );
 
 -- Index for efficient snapshot lookups

internal/vaultik/snapshot.go

@@ -988,7 +988,6 @@ 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 {
@@ -1003,8 +1002,14 @@ func (v *Vaultik) listAllRemoteSnapshotIDs() ([]string, error) {
 			}
 			if strings.HasSuffix(object.Key, "/") || strings.Contains(object.Key, "/manifest.json.zst") {
 				sid := parts[1]
-				if !seen[sid] {
-					seen[sid] = true
+				found := false
+				for _, id := range snapshotIDs {
+					if id == sid {
+						found = true
+						break
+					}
+				}
+				if !found {
 					snapshotIDs = append(snapshotIDs, sid)
 				}
 			}