Backintrack systeem

Reminder voor Rene:
- Backings track zijn relaxed compatible voor akkoorden die erop lijken. (e.g. Cm7 in backing track wordt ook getoond op Cm9 in de oefening) (andersom niet)
- backing track systeem doet 2 voorstellen: 100% match en een mindere match op basis van alleen basis akkoord (7)

Akkoord info is nu in dit formaat:

```
Cm7:1;Cm7:1;Cm7:1;Cm7:1;Cm7:1;Cm7:1;Cm7:1;Cm7:1;
Dm7:1;Dm7:1;Dm7:1;Dm7:1;Dm7:1;Dm7:1;Dm7:1;Dm7:1;
Em7:1;Em7:1;Em7:1;Em7:1;Em7:1;Em7:1;Em7:1;Em7:1;
Fism7:1;Fism7:1;Fism7:1;Fism7:1;Fism7:1;Fism7:1;Fism7:1;Fism7:1;
Aesm7:1;Aesm7:1;Aesm7:1;Aesm7:1;Aesm7:1;Aesm7:1;Aesm7:1;Aesm7:1;
Besm7:1;Besm7:1;Besm7:1;Besm7:1;Besm7:1;Besm7:1;Besm7:1;Besm7:1;
```
Of voor multichord:

` Dm7:2;G7:2;Cmaj7:1 `

The issue names no files, tests, or entry points. First locate the backing-track matching logic in the Python/Flask app and clarify the chord-compatibility and two-proposal rules, including multichords. Done means the system distinguishes exact matches from root-chord matches and applies the stated one-way compatibility behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.