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

go.mod

@@ -37,6 +37,7 @@ require (
 	github.com/Azure/azure-sdk-for-go/sdk/keyvault/azsecrets v0.12.0 // indirect
 	github.com/Azure/azure-sdk-for-go/sdk/keyvault/internal v0.7.1 // indirect
 	github.com/AzureAD/microsoft-authentication-library-for-go v1.4.2 // indirect
+	github.com/adrg/xdg v0.5.3 // indirect
 	github.com/armon/go-metrics v0.4.1 // indirect
 	github.com/aws/aws-sdk-go v1.44.256 // indirect
 	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.11 // indirect