feat(server/m7-381): admin scan-status + manual trigger endpoints

Adds GET /api/admin/scan/status and POST /api/admin/scan/run under the
existing RequireAdmin middleware block; plumbs *library.Scanner and
library.RunScanConfig through api.Mount, server.New, and main.go so the
manual trigger reuses the same RunScan orchestrator as startup scans.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -144,9 +144,13 @@ func run() error {
 		}
 	}
 
+	scanCfg := library.RunScanConfig{
+		BackfillCap: 5000,
+		EnrichCap:   cfg.Library.CoverArtBackfillCap,
+	}
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap, scanner, scanCfg)
 	playlists.StartSystemPlaylistCron(ctx, pool, logger.With("component", "system_playlist_cron"))
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,