feat: the backup can include the music, sending each library file once to music/ and again only when it changes (#6774)

With "Include the music" on, each night's run sends the library files the
destination doesn't hold yet, after the database file has landed, for up to
six hours; a large library spans several nights the first time. Files keep
their path under the library folders' common root, beside database/ in the
same folder or bucket.

backup_music_files records each file's size and mtime as sent, so a night
sends only new and changed files, and a rescan that touches a track but finds
the same file sends nothing. A changed file goes to its old name; Minstrel
never deletes there. An object the bucket refuses (a locked overwrite) is
kept with its answer and goes again once the file changes or the settings are
saved; three refusals in a row stop the night's music, since that is a key
that may not write under music/. A folder copy is written under a .part name
and renamed, keeping the file's time.

The panel shows how much of the library is in, the refused files, and each
run's music count and note. The verdict still turns on the database file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/hosting.md

@@ -102,6 +102,21 @@ Minstrel only adds files, named by date, and never deletes one. Keep as many
 as you like with the destination's own retention. **Test destination** lists
 what is there without writing anything.
 
+**Include the music** also sends the library's files to `music/` in the same
+destination, laid out as they are under the library folders. After each
+night's database file has landed, Minstrel sends the files it hasn't sent
+yet, one at a time, for up to six hours; a large library takes several
+nights the first time, and the panel shows how much is in. After that, a
+night sends only new files and files that changed. A changed file goes to
+its old name, and Minstrel still never deletes anything there, so a file
+taken out of the library stays in the backup. On a bucket that keeps
+versions or locks objects, set its retention to keep what you want. A file
+the bucket refuses (an overwrite a locked object won't take) is listed on
+the panel and goes again once it changes or the settings are saved; three
+refusals in a row stop that night's music, since that is usually a key that
+may not write under `music/`. A bucket's upload limit applies
+to the music as well.
+
 ## Android app
 
 The app connects over whatever URL you give it, HTTP or HTTPS. Once the