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/module.go

@@ -3,7 +3,7 @@ package snapshot
 import (
 	"git.eeqj.de/sneak/vaultik/internal/config"
 	"git.eeqj.de/sneak/vaultik/internal/database"
-	"git.eeqj.de/sneak/vaultik/internal/s3"
+	"git.eeqj.de/sneak/vaultik/internal/storage"
 	"github.com/spf13/afero"
 	"go.uber.org/fx"
 )
@@ -27,13 +27,13 @@ var Module = fx.Module("backup",
 // ScannerFactory creates scanners with custom parameters
 type ScannerFactory func(params ScannerParams) *Scanner
 
-func provideScannerFactory(cfg *config.Config, repos *database.Repositories, s3Client *s3.Client) ScannerFactory {
+func provideScannerFactory(cfg *config.Config, repos *database.Repositories, storer storage.Storer) ScannerFactory {
 	return func(params ScannerParams) *Scanner {
 		return NewScanner(ScannerConfig{
 			FS:               params.Fs,
 			ChunkSize:        cfg.ChunkSize.Int64(),
 			Repositories:     repos,
-			S3Client:         s3Client,
+			Storage:          storer,
 			MaxBlobSize:      cfg.BlobSizeLimit.Int64(),
 			CompressionLevel: cfg.CompressionLevel,
 			AgeRecipients:    cfg.AgeRecipients,