feat(server): targeted ScanFiles + fsnotify library watcher

Add Scanner.ScanFiles (watcher-driven targeted scan returning changed album
IDs) and a recursive fsnotify Watcher that debounces filesystem events and
enriches just the affected albums inline. Pure classifyEvent/drainPending
seams unit-tested; ScanFiles covered in the scanner integration test.

internal/library/watcher_test.go

@@ -0,0 +1,59 @@
+package library
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+func TestClassifyEvent(t *testing.T) {
+	tests := []struct {
+		name    string
+		op      fsnotify.Op
+		isDir   bool
+		isAudio bool
+		want    watchAction
+	}{
+		{"create audio file enqueues", fsnotify.Create, false, true, watchEnqueueFile},
+		{"write audio file enqueues", fsnotify.Write, false, true, watchEnqueueFile},
+		{"create non-audio file ignored", fsnotify.Create, false, false, watchIgnore},
+		{"create directory scans it", fsnotify.Create, true, false, watchScanDir},
+		{"write on directory ignored", fsnotify.Write, true, false, watchIgnore},
+		{"remove audio file ignored (no pruning)", fsnotify.Remove, false, true, watchIgnore},
+		{"rename audio file ignored", fsnotify.Rename, false, true, watchIgnore},
+		{"chmod audio file ignored", fsnotify.Chmod, false, true, watchIgnore},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := classifyEvent(tt.op, tt.isDir, tt.isAudio); got != tt.want {
+				t.Fatalf("classifyEvent(%v, dir=%v, audio=%v) = %v, want %v",
+					tt.op, tt.isDir, tt.isAudio, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestDrainPending(t *testing.T) {
+	t.Run("empty set returns nil", func(t *testing.T) {
+		if got := drainPending(map[string]struct{}{}); got != nil {
+			t.Fatalf("drainPending(empty) = %v, want nil", got)
+		}
+	})
+
+	t.Run("returns paths and clears the set", func(t *testing.T) {
+		pending := map[string]struct{}{
+			"/music/a.mp3": {},
+			"/music/b.flac": {},
+		}
+		got := drainPending(pending)
+		sort.Strings(got)
+		want := []string{"/music/a.mp3", "/music/b.flac"}
+		if len(got) != len(want) || got[0] != want[0] || got[1] != want[1] {
+			t.Fatalf("drainPending = %v, want %v", got, want)
+		}
+		if len(pending) != 0 {
+			t.Fatalf("pending not cleared: %v", pending)
+		}
+	})
+}