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/snapshot/snapshot.go

@@ -52,7 +52,7 @@ import (
 	"git.eeqj.de/sneak/vaultik/internal/config"
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/log"
-	"git.eeqj.de/sneak/vaultik/internal/s3"
+	"git.eeqj.de/sneak/vaultik/internal/storage"
 	"github.com/dustin/go-humanize"
 	"github.com/spf13/afero"
 	"go.uber.org/fx"
@@ -60,27 +60,27 @@ import (
 
 // SnapshotManager handles snapshot creation and metadata export
 type SnapshotManager struct {
-	repos    *database.Repositories
-	s3Client S3Client
-	config   *config.Config
-	fs       afero.Fs
+	repos   *database.Repositories
+	storage storage.Storer
+	config  *config.Config
+	fs      afero.Fs
 }
 
 // SnapshotManagerParams holds dependencies for NewSnapshotManager
 type SnapshotManagerParams struct {
 	fx.In
 
-	Repos    *database.Repositories
-	S3Client *s3.Client
-	Config   *config.Config
+	Repos   *database.Repositories
+	Storage storage.Storer
+	Config  *config.Config
 }
 
 // NewSnapshotManager creates a new snapshot manager for dependency injection
 func NewSnapshotManager(params SnapshotManagerParams) *SnapshotManager {
 	return &SnapshotManager{
-		repos:    params.Repos,
-		s3Client: params.S3Client,
-		config:   params.Config,
+		repos:   params.Repos,
+		storage: params.Storage,
+		config:  params.Config,
 	}
 }
 
@@ -268,7 +268,7 @@ func (sm *SnapshotManager) ExportSnapshotMetadata(ctx context.Context, dbPath st
 	dbKey := fmt.Sprintf("metadata/%s/db.zst.age", snapshotID)
 
 	dbUploadStart := time.Now()
-	if err := sm.s3Client.PutObject(ctx, dbKey, bytes.NewReader(finalData)); err != nil {
+	if err := sm.storage.Put(ctx, dbKey, bytes.NewReader(finalData)); err != nil {
 		return fmt.Errorf("uploading snapshot database: %w", err)
 	}
 	dbUploadDuration := time.Since(dbUploadStart)
@@ -282,7 +282,7 @@ func (sm *SnapshotManager) ExportSnapshotMetadata(ctx context.Context, dbPath st
 	// Upload blob manifest (compressed only, not encrypted)
 	manifestKey := fmt.Sprintf("metadata/%s/manifest.json.zst", snapshotID)
 	manifestUploadStart := time.Now()
-	if err := sm.s3Client.PutObject(ctx, manifestKey, bytes.NewReader(blobManifest)); err != nil {
+	if err := sm.storage.Put(ctx, manifestKey, bytes.NewReader(blobManifest)); err != nil {
 		return fmt.Errorf("uploading blob manifest: %w", err)
 	}
 	manifestUploadDuration := time.Since(manifestUploadStart)
@@ -635,11 +635,11 @@ func (sm *SnapshotManager) CleanupIncompleteSnapshots(ctx context.Context, hostn
 
 	log.Info("Found incomplete snapshots", "count", len(incompleteSnapshots))
 
-	// Check each incomplete snapshot for metadata in S3
+	// Check each incomplete snapshot for metadata in storage
 	for _, snapshot := range incompleteSnapshots {
-		// Check if metadata exists in S3
+		// Check if metadata exists in storage
 		metadataKey := fmt.Sprintf("metadata/%s/db.zst", snapshot.ID)
-		_, err := sm.s3Client.StatObject(ctx, metadataKey)
+		_, err := sm.storage.Stat(ctx, metadataKey)
 
 		if err != nil {
 			// Metadata doesn't exist in S3 - this is an incomplete snapshot