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/vaultik.go

@@ -10,8 +10,8 @@ import (
 	"git.eeqj.de/sneak/vaultik/internal/crypto"
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/globals"
-	"git.eeqj.de/sneak/vaultik/internal/s3"
 	"git.eeqj.de/sneak/vaultik/internal/snapshot"
+	"git.eeqj.de/sneak/vaultik/internal/storage"
 	"github.com/spf13/afero"
 	"go.uber.org/fx"
 )
@@ -22,7 +22,7 @@ type Vaultik struct {
 	Config          *config.Config
 	DB              *database.DB
 	Repositories    *database.Repositories
-	S3Client        *s3.Client
+	Storage         storage.Storer
 	ScannerFactory  snapshot.ScannerFactory
 	SnapshotManager *snapshot.SnapshotManager
 	Shutdowner      fx.Shutdowner
@@ -46,7 +46,7 @@ type VaultikParams struct {
 	Config          *config.Config
 	DB              *database.DB
 	Repositories    *database.Repositories
-	S3Client        *s3.Client
+	Storage         storage.Storer
 	ScannerFactory  snapshot.ScannerFactory
 	SnapshotManager *snapshot.SnapshotManager
 	Shutdowner      fx.Shutdowner
@@ -72,7 +72,7 @@ func New(params VaultikParams) *Vaultik {
 		Config:          params.Config,
 		DB:              params.DB,
 		Repositories:    params.Repositories,
-		S3Client:        params.S3Client,
+		Storage:         params.Storage,
 		ScannerFactory:  params.ScannerFactory,
 		SnapshotManager: params.SnapshotManager,
 		Shutdowner:      params.Shutdowner,