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/app.go

@@ -2,9 +2,11 @@ package cli
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"syscall"
 	"time"
 
@@ -12,9 +14,11 @@ 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/pidlock"
 	"git.eeqj.de/sneak/vaultik/internal/snapshot"
+	"git.eeqj.de/sneak/vaultik/internal/storage"
 	"git.eeqj.de/sneak/vaultik/internal/vaultik"
+	"github.com/adrg/xdg"
 	"go.uber.org/fx"
 )
 
@@ -51,7 +55,7 @@ func NewApp(opts AppOptions) *fx.App {
 		config.Module,
 		database.Module,
 		log.Module,
-		s3.Module,
+		storage.Module,
 		snapshot.Module,
 		fx.Provide(vaultik.New),
 		fx.Invoke(setupGlobals),
@@ -118,7 +122,23 @@ func RunApp(ctx context.Context, app *fx.App) error {
 // RunWithApp is a helper that creates and runs an fx app with the given options.
 // It combines NewApp and RunApp into a single convenient function. This is the
 // preferred way to run CLI commands that need the full application context.
+// It acquires a PID lock before starting to prevent concurrent instances.
 func RunWithApp(ctx context.Context, opts AppOptions) error {
+	// Acquire PID lock to prevent concurrent instances
+	lockDir := filepath.Join(xdg.DataHome, "berlin.sneak.app.vaultik")
+	lock, err := pidlock.Acquire(lockDir)
+	if err != nil {
+		if errors.Is(err, pidlock.ErrAlreadyRunning) {
+			return fmt.Errorf("cannot start: %w", err)
+		}
+		return fmt.Errorf("failed to acquire lock: %w", err)
+	}
+	defer func() {
+		if err := lock.Release(); err != nil {
+			log.Warn("Failed to release PID lock", "error", err)
+		}
+	}()
+
 	app := NewApp(opts)
 	return RunApp(ctx, app)
 }