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>

README.md

@@ -20,13 +20,21 @@ path on both sides. The fallbacks below catch small differences, and a scan warn
 | Same title, **different format** (`Song.mp3` vs `Song.flac`) | Status `similar` (other format). Manual review. |
 | Same title after ignoring track numbers/punctuation (`01 - Song!.flac` vs `Song.mp3`) | Status `similar` (similar title). Manual review. |
 | Same top-level folder and title, but a **different subfolder** | Status `similar` (same top folder, other subfolder). Manual review. |
+| *(opt-in)* **Approximately** the same title, disc folders ignored (`Cats/Disc 02/07 Memory.flac` vs `Cats/07 Memory.mp3`) | Status `similar` (approximate match), with the reason shown. Manual review. |
 | Only exists in the slave library | Status `unmatched`. Never touched. |
 | Already the same inode | Status `linked`. |
 
+**Approximate matching** is an opt-in setting on each pair. For slave files the strict rules could not place, it searches the same
+album with disc folders (`Disc 02`, `CD2`, …) ignored, then the whole top-level folder, and compares titles by similarity against a
+threshold you choose (85% by default; a matching track number adds a small bonus). Several guards keep it from linking the wrong song:
+version words such as *Live*, *Remix*, *Reprise* or *Instrumental* must agree on both sides, a file is left unmatched when two
+candidates are nearly tied, and approximate matches never auto-link.
+
 **Manual review** shows both files side by side and ranks their quality: lossless beats lossy, then bit depth and sample rate, then
 codec-weighted bitrate (Opus and AAC count for more per kbps than MP3). You then have three choices:
 
-- **Link to master copy.** The default. The master is kept because it belongs to a complete album. If the master is *lower* quality, playlink
+- **Upgrade / link to master copy.** The default. When the master copy is better, the match is flagged as an **upgrade** and listed
+  under the *Upgrades* filter. The master is kept because it belongs to a complete album. If the master is *lower* quality, playlink
   warns you and asks for explicit confirmation.
 - **Use slave copy as master.** The override. The slave's better file replaces the master file, the old master file is quarantined, and both
   libraries share the better file.
@@ -108,7 +116,8 @@ Run a scan, check the Review tab, and turn off dry run once the planned actions
 | `PLAYLINK_FFPROBE` | `ffprobe` | Path to ffprobe. Without it, quality comparison is disabled but everything else works. |
 | `PLAYLINK_LOG_LEVEL` | `info` | `debug` for verbose logs. |
 
-Per-pair settings (in the UI) are file extensions, auto-link, scan interval, quarantine folder, retention and settle time.
+Per-pair settings (in the UI) are file extensions, auto-link, approximate matching and its threshold, scan interval, quarantine folder,
+retention and settle time.
 
 ## Development