Add snapshot files to list a snapshot's files without restoring it (closes #435)

`vaultik snapshot files <snapshot-id> [path] [--json]` downloads and
decrypts the snapshot database, as restore does, and prints mode, size,
modification time and path for every file at or under the path. A path
that matches nothing is an error. It needs the private key. The mode is
in Go's file-mode notation, which the README spells out. The table
quotes a path that holds a non-printable character.

Named `files` because `ls` is already the alias of `snapshot list`.
The missing-key error no longer says "for restore", since this command
and deep verify return it too. The README opening now states the
private-key rule first.

Model: opus-5-5

internal/database/snapshots.go

@@ -238,10 +238,11 @@ func (r *SnapshotRepository) GetByID(
 
 // GetOnlySnapshot returns the sole snapshot in an exported per-snapshot
 // metadata database. The backup path writes each snapshot's database with
-// exactly one snapshot row (see cleanSnapshotDB), so restore and deep
-// verify expect exactly one. Zero rows return ErrNoSnapshotInDatabase and
-// more than one returns ErrMultipleSnapshotsInDatabase; callers treat
-// either as a failed identity check on the downloaded database.
+// exactly one snapshot row (see cleanSnapshotDB), so restore, deep verify
+// and `snapshot files` expect exactly one. Zero rows return
+// ErrNoSnapshotInDatabase and more than one returns
+// ErrMultipleSnapshotsInDatabase; callers treat either as a failed
+// identity check on the downloaded database.
 func (r *SnapshotRepository) GetOnlySnapshot(ctx context.Context) (*Snapshot, error) {
 	query := `
 		SELECT id, hostname, vaultik_version, vaultik_git_revision,