Add organizing files into Artist/Album/01 Title from their tags

A new Organize tab renames audio files from their tags into a per-pair
naming pattern, by default {artist}/{album}/{track} {title} (album
artist, falling back to artist). Multi-disc albums get a disc prefix
(2-04) or a Disc N folder; names are made safe for Windows/SMB shares.

- Preview first: every rename, files already organized, and files
  skipped with the reason (missing tags, with a button into the tag
  editor; two files resolving to the same name; an existing file).
- Applying re-checks each file against the preview, moves it within its
  library (hardlinks and contents untouched, case-only renames via a
  temporary name, completed on startup after a crash), carries .lrc
  lyrics and folder cover art along, and removes emptied folders. Each
  move is an operation that can be undone; the library is rescanned.
- Optional automatic organizing of the slave, master or both during
  scans (off by default). Dry run applies.
- Scans now recognise a slave file that is a hardlink of a master file
  at a different path as linked ("same file, other path") instead of
  unmatched.

Migration 7 adds the per-pair naming settings.

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

internal/engine/engine.go

@@ -43,6 +43,10 @@ type Engine struct {
 	mu sync.Mutex
 
 	tagTools tags.Tools
+	tagCache tagCache
+
+	plansMu sync.Mutex
+	plans   map[int64]*OrganizePlan // latest organize preview per pair
 
 	jobsMu sync.Mutex
 	jobs   map[int64]*Job
@@ -290,6 +294,9 @@ func (e *Engine) NormalizePair(p *store.Pair) ([]store.Warning, error) {
 	if p.RetentionDays < 0 || p.ScanIntervalMinutes < 0 || p.SettleMinutes < 0 {
 		return nil, errors.New("intervals must not be negative")
 	}
+	if err := normalizeOrganize(p); err != nil {
+		return nil, err
+	}
 	if p.ApproxThreshold == 0 {
 		p.ApproxThreshold = 85
 	}