feat(server): retire scan scheduler for watcher + safety-net walk

Wire the fsnotify watcher and a fixed 12h safety-net delta walk in main;
remove the configurable scan scheduler (scheduler.go, scan_schedule table via
migration 0033, GET/PATCH /api/admin/scan/schedule, and the server/api
plumbing). Manual scan + scan status are unchanged.

cmd/minstrel/main.go

@@ -5,12 +5,15 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"os"
 	"os/signal"
 	"syscall"
 	"time"
 
+	"github.com/jackc/pgx/v5/pgxpool"
+
 	"git.fabledsword.com/bvandeusen/minstrel/internal/config"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
@@ -53,6 +56,35 @@ func main() {
 	}
 }
 
+// safetyNetScanInterval backstops the fsnotify watcher with a low-frequency
+// full delta walk, catching anything the watcher missed (inotify watch-limit
+// exhaustion, dropped events). Fixed — there is no operator configuration.
+const safetyNetScanInterval = 12 * time.Hour
+
+// runSafetyNetScans ticks a delta RunScan at safetyNetScanInterval until ctx
+// is cancelled, deferring to TryStartScan's in-flight guard so it never
+// collides with a manual, startup, or watcher-driven scan.
+func runSafetyNetScans(ctx context.Context, pool *pgxpool.Pool, scanner *library.Scanner,
+	enricher *coverart.Enricher, logger *slog.Logger, scanCfg library.RunScanConfig,
+) {
+	ticker := time.NewTicker(safetyNetScanInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			started, existing, err := library.TryStartScan(ctx, pool, scanner, enricher, logger, scanCfg)
+			if err != nil {
+				logger.Error("safety-net scan: try start failed", "err", err)
+			} else if !started && existing != nil {
+				logger.Info("safety-net scan skipped — prior still in flight",
+					"in_flight_id", existing.ID)
+			}
+		}
+	}
+}
+
 func run() error {
 	configPath := flag.String("config", os.Getenv("MINSTREL_CONFIG"), "path to YAML config file")
 	flag.Parse()
@@ -237,12 +269,24 @@ func run() error {
 		ArtistEnrichCap: -1,
 		DataDir:         cfg.Storage.DataDir,
 	}
-	scheduler := library.NewScheduler(pool, logger.With("component", "scheduler"),
-		scanner, coverEnricher, scanCfg)
-	scheduler.Start(ctx)
+	// Filesystem watcher: near-instant pickup of new/changed files via
+	// fsnotify, scanning just the affected paths + enriching their albums
+	// inline. Replaces the removed configurable scan scheduler.
+	watcher := library.NewWatcher(scanner, coverEnricher,
+		logger.With("component", "watcher"), cfg.Library.ScanPaths)
+	go func() {
+		if werr := watcher.Run(ctx); werr != nil {
+			logger.Warn("library watcher exited", "err", werr)
+		}
+	}()
+	// Safety-net delta walk backstops the watcher (inotify limits / dropped
+	// events) at a fixed low frequency. No operator config.
+	go runSafetyNetScans(ctx, pool, scanner, coverEnricher,
+		logger.With("component", "scan_safetynet"), scanCfg)
+
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, coverSettings, scanner, scanCfg, scheduler)
+	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, coverSettings, scanner, scanCfg)
 	srv.Bus = bus
 	srv.PlaylistScheduler = playlistScheduler
 	srv.StreamSecret = cfg.StreamSecret