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/config.yml.example

@@ -23,6 +23,12 @@ general:
     # is rebuilt on the next start. Deleting it does throw away track analysis, which
     # is expensive to recompute.
     cache: .musicmouse-cache
+    # How many tracks the background analyzer may work on at once, each in its own
+    # worker process. Omitted means one per core bar one (capped at 8), which is what
+    # turns a first-time pass over a whole library from an overnight job into a coffee
+    # break on a desktop. Set it to 1 on a machine that has better things to do, or to
+    # a specific number to cap how much of it analysis may take.
+    # analysis_workers: 4
 
   # Serial port the ESP32 firmware is on. A dropped link is retried, not fatal.
   # Required - use "simulate" to run without the mouse attached, which is a complete