Add pluggable storage backend, PID locking, and improved scan progress

Storage backend:
- Add internal/storage package with Storer interface
- Implement FileStorer for local filesystem storage (file:// URLs)
- Implement S3Storer wrapping existing s3.Client
- Support storage_url config field (s3:// or file://)
- Migrate all consumers to use storage.Storer interface

PID locking:
- Add internal/pidlock package to prevent concurrent instances
- Acquire lock before app start, release on exit
- Detect stale locks from crashed processes

Scan progress improvements:
- Add fast file enumeration pass before stat() phase
- Use enumerated set for deletion detection (no extra filesystem access)
- Show progress with percentage, files/sec, elapsed time, and ETA
- Change "changed" to "changed/new" for clarity

Config improvements:
- Add tilde expansion for paths (~/)
- Use xdg library for platform-specific default index path

internal/vaultik/prune.go

@@ -21,9 +21,9 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 	allBlobsReferenced := make(map[string]bool)
 	manifestCount := 0
 
-	// List all snapshots in S3
+	// List all snapshots in storage
 	log.Info("Listing remote snapshots")
-	objectCh := v.S3Client.ListObjectsStream(v.ctx, "metadata/", false)
+	objectCh := v.Storage.ListStream(v.ctx, "metadata/")
 
 	var snapshotIDs []string
 	for object := range objectCh {
@@ -73,10 +73,10 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 
 	log.Info("Processed manifests", "count", manifestCount, "unique_blobs_referenced", len(allBlobsReferenced))
 
-	// List all blobs in S3
+	// List all blobs in storage
 	log.Info("Listing all blobs in storage")
 	allBlobs := make(map[string]int64) // hash -> size
-	blobObjectCh := v.S3Client.ListObjectsStream(v.ctx, "blobs/", true)
+	blobObjectCh := v.Storage.ListStream(v.ctx, "blobs/")
 
 	for object := range blobObjectCh {
 		if object.Err != nil {
@@ -136,7 +136,7 @@ func (v *Vaultik) PruneBlobs(opts *PruneOptions) error {
 	for i, hash := range unreferencedBlobs {
 		blobPath := fmt.Sprintf("blobs/%s/%s/%s", hash[:2], hash[2:4], hash)
 
-		if err := v.S3Client.RemoveObject(v.ctx, blobPath); err != nil {
+		if err := v.Storage.Delete(v.ctx, blobPath); err != nil {
 			log.Error("Failed to delete blob", "hash", hash, "error", err)
 			continue
 		}