Code-Cleanup: toter Check, none-Sentinel, Login-Jitter raus

- entry.go: redundantes stored != "" entfernt
- filepath-Sentinel "none" -> leerer String (Go-Zero-Value),
  Checks in entry.go/user.go und web/js/app.js vereinfacht
- auth.go: zufaelligen Login-Jitter samt loginJitter-Variable und
  Test-Seam entfernt (war nur Spielerei, kein echter Schutz)
- notes/migrations.md: einmalige "none"->"" Migration dokumentiert

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

user.go

@@ -94,7 +94,7 @@ func handleUserDelete(w http.ResponseWriter, r *http.Request) {
 	if rows, err := db.Query(`SELECT filepath FROM entry WHERE uid = ?`, uid); err == nil {
 		for rows.Next() {
 			var fp string
-			if rows.Scan(&fp) == nil && fp != "" && fp != "none" {
+			if rows.Scan(&fp) == nil && fp != "" {
 				media = append(media, fp)
 			}
 		}