fix(push): reinstalled app replaces its old subscription; devices listed and removable

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/push.go

@@ -148,6 +148,8 @@ func (s *Server) handlePushSubscribe(w http.ResponseWriter, r *http.Request) {
 			Auth   string `json:"auth"`
 		} `json:"keys"`
 		Device string `json:"device"`
+		// The subscription this browser had before (e.g. before a reinstall): it is dead now
+		Replaces string `json:"replaces"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültige Anfrage")
@@ -165,10 +167,14 @@ func (s *Server) handlePushSubscribe(w http.ResponseWriter, r *http.Request) {
 	if len(device) > 120 {
 		device = device[:120]
 	}
-	if err := s.store.SavePushSubscription(auth.GetUserID(r.Context()), req.Endpoint, req.Keys.P256dh, req.Keys.Auth, device); err != nil {
+	userID := auth.GetUserID(r.Context())
+	if err := s.store.SavePushSubscription(userID, req.Endpoint, req.Keys.P256dh, req.Keys.Auth, device); err != nil {
 		writeError(w, http.StatusInternalServerError, "Gerät konnte nicht gespeichert werden")
 		return
 	}
+	if req.Replaces != "" && req.Replaces != req.Endpoint {
+		_ = s.store.DeletePushSubscription(userID, req.Replaces) // only ever one of the own devices
+	}
 	writeJSON(w, http.StatusOK, map[string]bool{"ok": true})
 }