Add opt-in approximate matching and upgrade recommendations

A per-pair "Approximate matching" setting adds a second pass for slave
files the strict rules could not place. It searches the same album with
disc folders (Disc 02, CD2, ...) ignored, then the top-level folder, and
scores titles by edit distance and word containment against a configurable
threshold (default 85%), with a small track-number bonus.

Guards against false matches: version words (live, remix, reprise,
instrumental, ...) must agree, near-tied candidates are left unmatched,
and approximate matches always go to manual review, never auto-link.
Each match records a human-readable reason.

Review now flags matches where the master copy is better as upgrades,
with an Upgrades filter and count and an "Upgrade to master copy" action.
The redundant "master is lossless" warning note is dropped.

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

internal/store/store.go

@@ -104,6 +104,11 @@ var migrations = []string{
 	UPDATE operations SET quarantine_side = 'master' WHERE quarantine_side = 'host';
 	UPDATE matches SET verdict = replace(verdict, 'host', 'master');
 	UPDATE pairs SET last_scan_warnings = replace(last_scan_warnings, 'host', 'master');`,
+
+	// 3: opt-in approximate matching, and the reason shown for such matches.
+	`ALTER TABLE pairs ADD COLUMN approx_matching INTEGER NOT NULL DEFAULT 0;
+	ALTER TABLE pairs ADD COLUMN approx_threshold INTEGER NOT NULL DEFAULT 85;
+	ALTER TABLE matches ADD COLUMN reason TEXT NOT NULL DEFAULT '';`,
 }
 
 // Open opens (and migrates) the database in dataDir.