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.

go.mod

@@ -4,6 +4,7 @@ go 1.25.0
 
 require (
 	github.com/dhowden/tag v0.0.0-20240417053706-3d75831295e8
+	github.com/fsnotify/fsnotify v1.10.1
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/go-co-op/gocron/v2 v2.21.2
 	github.com/golang-migrate/migrate/v4 v4.19.1
@@ -25,5 +26,6 @@ require (
 	github.com/robfig/cron/v3 v3.0.1 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/sys v0.44.0 // indirect
 	golang.org/x/text v0.37.0 // indirect
 )