Go back to the hardlink deduplicator (1da0132), keeping backups

Restores the code of 1da0132 ("Add a setting to hide the quarantine from
Plex") and removes everything added after it: the tag editor, album art,
organizing/naming, the step-by-step tune-up and library types.

Kept:
- Database backups (Settings -> Backups): scheduled, manual, download,
  restore, and the optional backup before every action.
- The .env-driven docker-compose.yml for Arcane.

Databases that already ran the later migrations keep working: migrations
6-10 stay in the list (they only add columns) so numbering never goes
backwards, and Undo refuses History entries of removed features instead
of treating them as links.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/fsops/fsops.go

@@ -403,70 +403,3 @@ func CopyReplace(src, dst string, mode fs.FileMode) error {
 func MkdirAllLike(dir string) error {
 	return os.MkdirAll(dir, 0o775)
 }
-
-// CopyNew copies src to dst, which must not exist. The copy is fsynced and
-// verified byte for byte. Used for backups that must not share an inode with
-// the original (the original is about to be modified in place).
-func CopyNew(src, dst string, mode fs.FileMode) error {
-	in, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer in.Close()
-	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_EXCL, mode.Perm())
-	if err != nil {
-		return err
-	}
-	fail := func(e error) error { out.Close(); _ = os.Remove(dst); return e }
-	if _, err := io.Copy(out, in); err != nil {
-		return fail(err)
-	}
-	if err := out.Sync(); err != nil {
-		return fail(err)
-	}
-	if err := out.Close(); err != nil {
-		_ = os.Remove(dst)
-		return err
-	}
-	if same, err := SameContent(src, dst); err != nil || !same {
-		_ = os.Remove(dst)
-		if err == nil {
-			err = fmt.Errorf("%w: backup differs from the original", ErrVerifyFailed)
-		}
-		return err
-	}
-	return SyncDir(filepath.Dir(dst))
-}
-
-// OverwriteInPlace replaces the contents of dst with the bytes of src while
-// keeping dst's inode, so every hardlink of dst sees the new contents. This is
-// not atomic: callers keep a backup (CopyNew) to restore from on failure.
-func OverwriteInPlace(dst, src string) error {
-	in, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer in.Close()
-	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_TRUNC, 0)
-	if err != nil {
-		return err
-	}
-	if _, err := io.Copy(out, in); err != nil {
-		out.Close()
-		return err
-	}
-	if err := out.Sync(); err != nil {
-		out.Close()
-		return err
-	}
-	if err := out.Close(); err != nil {
-		return err
-	}
-	if same, err := SameContent(src, dst); err != nil || !same {
-		if err == nil {
-			err = fmt.Errorf("%w: file does not match what was written", ErrVerifyFailed)
-		}
-		return err
-	}
-	return nil
-}