Centralise field type defs (#6550)

### Summary

This PR eliminates duplicated field-to-type mappings across `Album` and
`Item` model classes. Previously, each model class maintained its own
`_fields: dict[str, types.Type]` — a verbose, hand-written dictionary
that encoded both _which_ fields exist and _what type_ each one has.
This duplication made it easy for the two definitions to drift out of
sync.

---

### Architecture Change

**Before**: Each model class owned a full inline `_fields` dict mapping
field names to type instances.

```
Album._fields = {"id": types.PRIMARY_ID, "artpath": types.NullPathType(), "added": types.DATE, ...}  # ~40 entries
Item._fields  = {"id": types.PRIMARY_ID, "path": types.PathType(), "album_id": types.FOREIGN_ID, ...}  # ~80 entries
```

**After**: Field names are declared as a `set[str]`, and the type
mapping is derived lazily from a single centralised `TYPE_BY_FIELD`
registry in `beets.dbcore.fields`.

```
Album._field_names = {"id", "artpath", "added", ...}        # plain set of names
Item._field_names  = Album._field_names - {"artpath"} | {...} # extends Album's set

LibModel._fields (cached_classproperty) = {f: TYPE_BY_FIELD[f] for f in cls._field_names}
```

This creates a clean separation of concerns:
- **What fields belong to a model** → declared in the model
(`_field_names`)
- **What type a field has** → declared once in
`dbcore.fields.TYPE_BY_FIELD`

---

### Key Impacts

| Area | Before | After |
|---|---|---|
| Type definitions | Inline per model | Single registry in
`dbcore.fields` |
| `Item._fields` | Fully independent dict | Derived from
`Album._field_names` |
| `Album.item_keys` | Hard-coded list of 40+ strings | `_field_names -
{"artpath", "id"}` |
| `_media_fields` / `_media_tag_fields` | Intersected with
`_fields.keys()` | Intersected with `_field_names` directly |
| Coverage measurement | `--cov=beets --cov=beetsplug` | `--cov=.`
(fixes missing `beetsplug/musicbrainz.py`) |

---

### Risk Areas to Review

- **`TYPE_BY_FIELD` completeness**: all field names in `_field_names`
must have a corresponding entry in the registry — a missing key will
raise at class definition time via `cached_classproperty`.
- **`Album.item_keys` semantics**: changed from `list` to `set` and is
now derived automatically. Any code that relied on ordering or specific
list membership should be checked.
- **`Item._field_names` inheritance expression**: `Album._field_names -
{"artpath"} | {...}` — operator precedence here is `(Album._field_names
- {"artpath"}) | {...}`, which is the intended behaviour, but worth a
close read.

Author: snejus

beets/library/fields.py

@@ -0,0 +1,102 @@
+from beets.dbcore import types
+
+TYPE_BY_FIELD: dict[str, types.Type] = {
+    "acoustid_fingerprint": types.STRING,
+    "acoustid_id": types.STRING,
+    "added": types.DATE,
+    "albumartist_credit": types.STRING,
+    "albumartists_credit": types.MULTI_VALUE_DSV,
+    "albumartist_sort": types.STRING,
+    "albumartists_sort": types.MULTI_VALUE_DSV,
+    "albumartists": types.MULTI_VALUE_DSV,
+    "albumartist": types.STRING,
+    "albumdisambig": types.STRING,
+    "album_id": types.FOREIGN_ID,
+    "albumstatus": types.STRING,
+    "album": types.STRING,
+    "albumtypes": types.SEMICOLON_SPACE_DSV,
+    "albumtype": types.STRING,
+    "arrangers": types.MULTI_VALUE_DSV,
+    "arrangers_ids": types.MULTI_VALUE_DSV,
+    "artist_credit": types.STRING,
+    "artists_credit": types.MULTI_VALUE_DSV,
+    "artists_ids": types.MULTI_VALUE_DSV,
+    "artist_sort": types.STRING,
+    "artists_sort": types.MULTI_VALUE_DSV,
+    "artists": types.MULTI_VALUE_DSV,
+    "artist": types.STRING,
+    "artpath": types.NullPathType(),
+    "asin": types.STRING,
+    "barcode": types.STRING,
+    "bitdepth": types.INTEGER,
+    "bitrate_mode": types.STRING,
+    "bitrate": types.ScaledInt(1000, "kbps"),
+    "bpm": types.INTEGER,
+    "catalognum": types.STRING,
+    "channels": types.INTEGER,
+    "comments": types.STRING,
+    "composer_sort": types.STRING,
+    "composers": types.MULTI_VALUE_DSV,
+    "composers_ids": types.MULTI_VALUE_DSV,
+    "comp": types.BOOLEAN,
+    "country": types.STRING,
+    "data_source": types.STRING,
+    "day": types.PaddedInt(2),
+    "discogs_albumid": types.INTEGER,
+    "discogs_artistid": types.INTEGER,
+    "discogs_labelid": types.INTEGER,
+    "disctitle": types.STRING,
+    "disctotal": types.PaddedInt(2),
+    "disc": types.PaddedInt(2),
+    "encoder_info": types.STRING,
+    "encoder_settings": types.STRING,
+    "encoder": types.STRING,
+    "format": types.STRING,
+    "genres": types.MULTI_VALUE_DSV,
+    "grouping": types.STRING,
+    "id": types.PRIMARY_ID,
+    "initial_key": types.MusicalKey(),
+    "isrc": types.STRING,
+    "label": types.STRING,
+    "language": types.STRING,
+    "length": types.DurationType(),
+    "lyricists": types.MULTI_VALUE_DSV,
+    "lyricists_ids": types.MULTI_VALUE_DSV,
+    "lyrics": types.STRING,
+    "mb_albumartistids": types.MULTI_VALUE_DSV,
+    "mb_albumartistid": types.STRING,
+    "mb_albumid": types.STRING,
+    "mb_artistids": types.MULTI_VALUE_DSV,
+    "mb_artistid": types.STRING,
+    "mb_releasegroupid": types.STRING,
+    "mb_releasetrackid": types.STRING,
+    "mb_trackid": types.STRING,
+    "mb_workid": types.STRING,
+    "media": types.STRING,
+    "month": types.PaddedInt(2),
+    "mtime": types.DATE,
+    "original_day": types.PaddedInt(2),
+    "original_month": types.PaddedInt(2),
+    "original_year": types.PaddedInt(4),
+    "path": types.PathType(),
+    "r128_album_gain": types.NULL_FLOAT,
+    "r128_track_gain": types.NULL_FLOAT,
+    "releasegroupdisambig": types.STRING,
+    "release_group_title": types.STRING,
+    "remixers": types.MULTI_VALUE_DSV,
+    "remixers_ids": types.MULTI_VALUE_DSV,
+    "rg_album_gain": types.NULL_FLOAT,
+    "rg_album_peak": types.NULL_FLOAT,
+    "rg_track_gain": types.NULL_FLOAT,
+    "rg_track_peak": types.NULL_FLOAT,
+    "samplerate": types.ScaledInt(1000, "kHz"),
+    "script": types.STRING,
+    "style": types.STRING,
+    "title": types.STRING,
+    "trackdisambig": types.STRING,
+    "tracktotal": types.PaddedInt(2),
+    "track": types.PaddedInt(2),
+    "work_disambig": types.STRING,
+    "work": types.STRING,
+    "year": types.PaddedInt(4),
+}