feat(auth): server-side sessions and current password for account changes (FLAW-30, FLAW-31)

- Every login is a session (device, IP, last use); the token's jti is its ID.
  Sessions end on logout, per device from the profile, or after 30 idle days.
- Tokens older than a day are renewed via the X-Session-Token header.
- Changing password or login e-mail needs the current password; a new
  password logs out all other devices and mails the old address.
- Demo reset keeps the demo visitors' sessions.
- revoked_tokens is replaced by sessions (migration 4); everyone is logged
  out once after the upgrade.

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

internal/api/handlers_auth.go

@@ -145,7 +145,7 @@ func (s *Server) handleFirstBootSetup(w http.ResponseWriter, r *http.Request) {
 		_ = s.store.SeedStarterContent(user)
 	}
 
-	token, err := auth.GenerateToken(s.cfg.JWTSecret, user.ID, user.Email, user.DisplayName, user.TokenVersion)
+	token, err := s.startSession(r, user)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "Token konnte nicht erstellt werden")
 		return
@@ -229,7 +229,7 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	token, err := auth.GenerateToken(s.cfg.JWTSecret, user.ID, user.Email, user.DisplayName, user.TokenVersion)
+	token, err := s.startSession(r, user)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "Token konnte nicht erstellt werden")
 		return
@@ -297,7 +297,7 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	token, err := auth.GenerateToken(s.cfg.JWTSecret, user.ID, user.Email, user.DisplayName, user.TokenVersion)
+	token, err := s.startSession(r, user)
 	if err != nil {
 		slog.Error("Login-Token konnte nicht erstellt werden", "login", identifier, "user_id", user.ID, "error", err)
 		writeError(w, http.StatusInternalServerError, "Token konnte nicht erstellt werden")
@@ -393,21 +393,11 @@ func importUpload(w http.ResponseWriter, r *http.Request) ([]byte, string, error
 
 func (s *Server) handleLogout(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
-	authHeader := r.Header.Get("Authorization")
-	if strings.HasPrefix(authHeader, "Bearer ") {
-		tokenStr := strings.TrimPrefix(authHeader, "Bearer ")
-		if claims, err := auth.ValidateToken(s.cfg.JWTSecret, tokenStr); err == nil && claims != nil {
-			var exp time.Time
-			if claims.ExpiresAt != nil {
-				exp = claims.ExpiresAt.Time
-			} else {
-				exp = time.Now().Add(30 * 24 * time.Hour)
-			}
-			// The session must really end; the client is told if it did not
-			if err := s.store.RevokeToken(tokenStr, userID, exp); err != nil {
-				writeError(w, http.StatusInternalServerError, "Abmelden fehlgeschlagen")
-				return
-			}
+	// The session must really end; the client is told if it did not
+	if sid := sessionIDFrom(r); sid != "" {
+		if err := s.store.RevokeSession(userID, sid); err != nil && !errors.Is(err, storage.ErrSessionNotFound) {
+			writeError(w, http.StatusInternalServerError, "Abmelden fehlgeschlagen")
+			return
 		}
 	}
 	// Streams are not tied to one session; other devices reconnect with a new ticket on their own
@@ -437,11 +427,23 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 		DiscordWebhookURL   *string   `json:"discordWebhookUrl"`
 		CustomWebhookURL    *string   `json:"customWebhookUrl"`
 		NotifyBeforeMinutes *int      `json:"notifyBeforeMinutes"`
+		// Needed to change password or e-mail address (FLAW-30)
+		CurrentPassword string `json:"currentPassword"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültiges JSON")
 		return
 	}
+	before, err := s.store.GetUserByID(userID)
+	if err != nil || before == nil {
+		writeError(w, http.StatusNotFound, "Benutzer nicht gefunden")
+		return
+	}
+	changesPassword := req.Password != nil && strings.TrimSpace(*req.Password) != ""
+	changesEmail := req.Email != nil && !strings.EqualFold(strings.TrimSpace(*req.Email), before.Email)
+	if (changesPassword || changesEmail) && !s.checkCurrentPassword(w, r, before, req.CurrentPassword) {
+		return
+	}
 	// Alarm URLs are checked before they are stored (SSRF protection, see netguard.go)
 	allowPrivate := s.allowPrivateWebhooks()
 	if req.DiscordWebhookURL != nil {
@@ -470,7 +472,18 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 			writeError(w, http.StatusInternalServerError, "Passwort konnte nicht gespeichert werden")
 			return
 		}
+		// Every other device is logged out; this one keeps its session with a fresh token
+		sid := sessionIDFrom(r)
+		if err := s.store.RevokeOtherSessions(userID, sid); err != nil {
+			writeError(w, http.StatusInternalServerError, "Andere Sitzungen konnten nicht beendet werden")
+			return
+		}
 		s.wsHub.DisconnectUser(userID)
+		if fresh, err := s.store.GetUserByID(userID); err == nil {
+			current := &auth.Claims{}
+			current.ID = sid
+			s.renewSessionToken(w, fresh, current, true)
+		}
 	}
 	u, err := s.store.UpdateUserFull(
 		userID,
@@ -489,9 +502,51 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 		writeError(w, http.StatusBadRequest, err.Error())
 		return
 	}
+	if changesPassword || changesEmail {
+		s.notifyAccountChange(before, changesPassword, changesEmail, u.Email)
+	}
 	writeJSON(w, http.StatusOK, u)
 }
 
+// checkCurrentPassword verifies the current password for sensitive changes; wrong attempts count
+// towards the login protection, so this cannot be used to guess the password.
+func (s *Server) checkCurrentPassword(w http.ResponseWriter, r *http.Request, user *domain.User, password string) bool {
+	ip := clientIPFrom(r)
+	if wait := s.loginGuard.lockedFor(user.ID, ip); wait > 0 {
+		writeLoginLocked(w, wait)
+		return false
+	}
+	// A forgotten field is no guess and does not count as a failure
+	if password == "" || !auth.CheckPassword(password, user.PasswordHash) {
+		if password != "" {
+			s.loginGuard.recordFailure(user.ID, ip)
+		}
+		writeError(w, http.StatusForbidden, "Bitte gib zur Bestätigung dein aktuelles Passwort richtig ein")
+		return false
+	}
+	return true
+}
+
+// notifyAccountChange tells the previous address about a new password or e-mail address, so a
+// takeover does not go unnoticed. Only when mail is set up; errors are logged, never shown.
+func (s *Server) notifyAccountChange(before *domain.User, password, email bool, newEmail string) {
+	if _, err := s.smtpSettings(); err != nil || before.Email == "" {
+		return
+	}
+	var what []string
+	if password {
+		what = append(what, "dein Passwort wurde geändert")
+	}
+	if email {
+		what = append(what, "die E-Mail-Adresse deines Kontos wurde auf "+newEmail+" geändert")
+	}
+	body := fmt.Sprintf("Hallo %s,\n\nin MindSift %s.\n\nWarst du das nicht, setze bitte sofort dein Passwort zurück und beende alle Sitzungen in den Kontoeinstellungen, oder wende dich an die Administration.\n",
+		before.DisplayName, strings.Join(what, " und "))
+	go func() {
+		_ = s.sendEmailNotification(before.Email, "MindSift: Änderung an deinem Konto", body)
+	}()
+}
+
 func (s *Server) handleTestUserAlarm(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
 	user, err := s.store.GetUserByID(userID)