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/api/api.go

@@ -15,6 +15,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/acquire"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/backup"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/catalogue"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/config"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
@@ -40,7 +41,7 @@ import (
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, requestSvc *requests.Service, quarantineSvc *quarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService, loudSettings *library.LoudnessSettingsService, acoustIDLookup *library.AcoustIDLookupWorker) {
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig, recSettings *recsettings.Service, requestSvc *requests.Service, quarantineSvc *quarantine.Service, tracksSvc *tracks.Service, playlistsSvc *playlists.Service, coverEnricher *coverart.Enricher, coverSettings *coverart.SettingsService, tagSettings *tags.SettingsService, scanner *library.Scanner, scanCfg library.RunScanConfig, dataDir string, sender mailer.Sender, bus *eventbus.Bus, playlistScheduler *playlists.Scheduler, streamSecret []byte, netSettings *netsettings.Service, reacqSettings *reacquisition.SettingsService, fpSettings *library.FingerprintSettingsService, loudSettings *library.LoudnessSettingsService, acoustIDLookup *library.AcoustIDLookupWorker, backups *backup.Service) {
 	rng := rand.New(rand.NewSource(rand.Int63()))
 	setupToken, err := auth.NewSetupToken()
 	if err != nil {
@@ -74,6 +75,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		fingerprintSettings: fpSettings,
 		loudnessSettings:    loudSettings,
 		acoustIDLookup:      acoustIDLookup,
+		backups:             backups,
 		acquisition:         acquire.NewStore(pool),
 		catalogue:           cat,
 		wanted:              wanted.New(pool, cat, logger),
@@ -358,6 +360,11 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Post("/tag-sources/{provider_id}/test", h.handleTestTagSource)
 				admin.Post("/tag-sources/research", h.handleResearchTags)
 
+				admin.Get("/backups", h.handleGetBackups)
+				admin.Put("/backups", h.handleUpdateBackups)
+				admin.Post("/backups/test", h.handleTestBackups)
+				admin.Post("/backups/run", h.handleRunBackup)
+
 				admin.Get("/smtp-config", h.handleGetSMTPConfig)
 				admin.Put("/smtp-config", h.handleUpdateSMTPConfig)
 				admin.Post("/smtp-config/test", h.handleTestSMTPConfig)
@@ -429,6 +436,8 @@ type handlers struct {
 	// acoustIDLookup is the AcoustID lookup worker: its settings, its
 	// status and "look up now". Nil serves the defaults (off).
 	acoustIDLookup *library.AcoustIDLookupWorker
+	// backups is the server's own backup: settings, status, test and run.
+	backups *backup.Service
 	// acquisition is the configured indexers and download clients.
 	acquisition *acquire.Store
 	// catalogue is the release catalogue read from MusicBrainz, and