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/cli/restore.go

@@ -8,8 +8,8 @@ import (
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/globals"
 	"git.eeqj.de/sneak/vaultik/internal/log"
-	"git.eeqj.de/sneak/vaultik/internal/s3"
 	"git.eeqj.de/sneak/vaultik/internal/snapshot"
+	"git.eeqj.de/sneak/vaultik/internal/storage"
 	"github.com/spf13/cobra"
 	"go.uber.org/fx"
 )
@@ -24,7 +24,7 @@ type RestoreApp struct {
 	Globals      *globals.Globals
 	Config       *config.Config
 	Repositories *database.Repositories
-	S3Client     *s3.Client
+	Storage      storage.Storer
 	DB           *database.DB
 	Shutdowner   fx.Shutdowner
 }
@@ -61,15 +61,14 @@ The age_secret_key must be configured in the config file for decryption.`,
 				},
 				Modules: []fx.Option{
 					snapshot.Module,
-					s3.Module,
 					fx.Provide(fx.Annotate(
 						func(g *globals.Globals, cfg *config.Config, repos *database.Repositories,
-							s3Client *s3.Client, db *database.DB, shutdowner fx.Shutdowner) *RestoreApp {
+							storer storage.Storer, db *database.DB, shutdowner fx.Shutdowner) *RestoreApp {
 							return &RestoreApp{
 								Globals:      g,
 								Config:       cfg,
 								Repositories: repos,
-								S3Client:     s3Client,
+								Storage:      storer,
 								DB:           db,
 								Shutdowner:   shutdowner,
 							}