Analyse tracks in a pool of worker processes

A first pass over an unanalysed library is hours of librosa, and there was no
reason for a desktop to spend them one core at a time. Analysis now runs in a
process pool sized by `general.library.analysis_workers`, defaulting to one per
core bar one (capped at 8) when the key is absent.

Two consequences worth knowing before touching this: whatever an `Analyzer`
returns has to be picklable, and an analyzer that keeps state on its own
instance - a test double counting calls - only behaves as written with
`analysis_workers=1`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

python-backend/musicmouse/config.py

@@ -172,6 +172,11 @@ class LibraryConfig(_Strict):
     root: Path
     #: Scan results, extracted cover art and track analysis. Relative to this file.
     cache: Path = Path(".musicmouse-cache")
+    #: How many tracks background analysis may work on at once, each in its own worker
+    #: process. Omit for one per core bar one (see
+    #: :func:`musicmouse.library.workers.default_worker_count`); set it to 1 to keep
+    #: analysis to a single process on a machine that has other work to do.
+    analysis_workers: int | None = Field(default=None, ge=1)
 
     @field_validator("root")
     @classmethod