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/storage/storer.go

@@ -0,0 +1,74 @@
+// Package storage provides a unified interface for storage backends.
+// It supports both S3-compatible object storage and local filesystem storage,
+// allowing Vaultik to store backups in either location with the same API.
+//
+// Storage backends are selected via URL:
+//   - s3://bucket/prefix?endpoint=host&region=r - S3-compatible storage
+//   - file:///path/to/backup - Local filesystem storage
+//
+// Both backends implement the Storer interface and support progress reporting
+// during upload/write operations.
+package storage
+
+import (
+	"context"
+	"errors"
+	"io"
+)
+
+// ErrNotFound is returned when an object does not exist.
+var ErrNotFound = errors.New("object not found")
+
+// ProgressCallback is called during storage operations with bytes transferred so far.
+// Return an error to cancel the operation.
+type ProgressCallback func(bytesTransferred int64) error
+
+// ObjectInfo contains metadata about a stored object.
+type ObjectInfo struct {
+	Key  string // Object key/path
+	Size int64  // Size in bytes
+	Err  error  // Error for streaming results (nil on success)
+}
+
+// StorageInfo provides human-readable storage configuration.
+type StorageInfo struct {
+	Type     string // "s3" or "file"
+	Location string // endpoint/bucket for S3, base path for filesystem
+}
+
+// Storer defines the interface for storage backends.
+// All paths are relative to the storage root (bucket/prefix for S3, base directory for filesystem).
+type Storer interface {
+	// Put stores data at the specified key.
+	// Parent directories are created automatically for filesystem backends.
+	Put(ctx context.Context, key string, data io.Reader) error
+
+	// PutWithProgress stores data with progress reporting.
+	// Size must be the exact size of the data to store.
+	// The progress callback is called periodically with bytes transferred.
+	PutWithProgress(ctx context.Context, key string, data io.Reader, size int64, progress ProgressCallback) error
+
+	// Get retrieves data from the specified key.
+	// The caller must close the returned ReadCloser.
+	// Returns ErrNotFound if the object does not exist.
+	Get(ctx context.Context, key string) (io.ReadCloser, error)
+
+	// Stat returns metadata about an object without retrieving its contents.
+	// Returns ErrNotFound if the object does not exist.
+	Stat(ctx context.Context, key string) (*ObjectInfo, error)
+
+	// Delete removes an object. No error is returned if the object doesn't exist.
+	Delete(ctx context.Context, key string) error
+
+	// List returns all keys with the given prefix.
+	// For large result sets, prefer ListStream.
+	List(ctx context.Context, prefix string) ([]string, error)
+
+	// ListStream returns a channel of ObjectInfo for large result sets.
+	// The channel is closed when listing completes.
+	// If an error occurs during listing, the final item will have Err set.
+	ListStream(ctx context.Context, prefix string) <-chan ObjectInfo
+
+	// Info returns human-readable storage location information.
+	Info() StorageInfo
+}