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
