feat: Minstrel backs itself up nightly to a folder or an S3 bucket, off until an admin switches it on (#6773)

Admin › Backups shows a verdict first (off, not set up, waiting, backed up,
overdue, failing since when), Back up now, Test destination and the latest
runs, with the settings beside them.

- backup.Service: one run at a time, bounded at 30 minutes; the schedule
  ticks every 5 minutes, catches up a slot missed while the server was down,
  and retries a failure after an hour and then after as long as it has been
  failing. Runs are recorded in backup_runs, pruned after 90 days; a run cut
  off by a shutdown is marked failed at the next start.
- A folder destination writes database/minstrel-<UTC stamp>.tar and refuses
  a folder that isn't there. An S3 destination stages the file, hashes it,
  and uploads it under database/ through a throttled reader (4 MiB/s by
  default, 0 for no limit).
- Test destination checks the form as typed and only lists.
- The bucket secret is sealed like every stored credential; backup_settings
  and backup_runs are left out of the backup itself.
- The panel polls only while a run is going, capped at the run limit plus
  five minutes.

The music switch named in the task is left to #6774, so no control ships
that does nothing.

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

internal/server/server.go

@@ -15,6 +15,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/api"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/backup"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/buildinfo"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/config"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
@@ -94,6 +95,9 @@ type Server struct {
 	// the admin card reaches it without a restart. Router() constructs a
 	// fallback when nil (tests).
 	ReacqSettings *reacquisition.SettingsService
+	// Backups is the server's own backup, the instance whose loop runs the
+	// nightly schedule. Nil builds one that runs only when an admin asks.
+	Backups *backup.Service
 	// StreamSecret is the HMAC key used by /api/cast/stream-token to
 	// mint signed UPnP / Sonos stream URLs and by /api/tracks/{id}/stream
 	// to verify them. Sourced from config.Config.StreamSecret. Tests that
@@ -200,7 +204,11 @@ func (s *Server) Router() http.Handler {
 			}
 			acoustIDLookup = library.NewAcoustIDLookupWorker(s.Pool, s.Logger, acoustIDSettings, nil)
 		}
-		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, recSettings, requestSvc, quarantineSvc, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.TagSettings, s.LibraryScanner, s.ScanCfg, s.DataDir, smtpSender, bus, s.PlaylistScheduler, s.StreamSecret, netSettings, reacqSettings, fpSettings, loudSettings, acoustIDLookup)
+		backups := s.Backups
+		if backups == nil {
+			backups = backup.NewService(s.Pool, s.Logger, buildinfo.Version)
+		}
+		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg, recSettings, requestSvc, quarantineSvc, tracksSvc, playlistsSvc, s.CoverEnricher, s.CoverSettings, s.TagSettings, s.LibraryScanner, s.ScanCfg, s.DataDir, smtpSender, bus, s.PlaylistScheduler, s.StreamSecret, netSettings, reacqSettings, fpSettings, loudSettings, acoustIDLookup, backups)
 		// /api/admin/scan is the only admin route owned by the server package
 		// (it needs the Scanner). Register it as a single inline-middleware
 		// route — using r.Route("/api/admin", ...) here would create a second