fix(PROJ-1): bcrypt Cost 12, Rate-Limiting, last_login_at, User Update/Delete

- bcrypt cost erhöht von DefaultCost (10) auf 12
- Rate-Limiting: max 5 Fehlversuche in 15 Min → HTTP 429
- last_login_at in DB gespeichert und bei jedem Login aktualisiert
- login_attempts Tabelle für Fehlversuche
- PATCH /api/users/{id}: Passwort-Reset, Rolle, E-Mail, Active
- DELETE /api/users/{id}: Löschen mit Schutz für letzten Admin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/server.go

@@ -88,6 +88,8 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("POST /api/auth/logout", s.authMiddleware(s.handleLogout))
 	s.mux.HandleFunc("GET /api/users", s.authMiddleware(s.requireRole(userstore.RoleAdmin, s.handleListUsers)))
 	s.mux.HandleFunc("POST /api/users", s.authMiddleware(s.requireRole(userstore.RoleAdmin, s.handleCreateUser)))
+	s.mux.HandleFunc("PATCH /api/users/{id}", s.authMiddleware(s.requireRole(userstore.RoleAdmin, s.handleUpdateUser)))
+	s.mux.HandleFunc("DELETE /api/users/{id}", s.authMiddleware(s.requireRole(userstore.RoleAdmin, s.handleDeleteUser)))
 	s.mux.HandleFunc("GET /api/search", s.authMiddleware(s.handleSearch))
 	s.mux.HandleFunc("GET /api/audit", s.authMiddleware(s.requireRole(userstore.RoleAuditor, s.handleAuditLog)))
 	s.mux.HandleFunc("GET /api/admin/smtp/status", s.authMiddleware(s.requireRole(userstore.RoleAdmin, s.handleSMTPStatus)))
@@ -124,6 +126,11 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
 }
 
+const (
+	loginMaxFailures = 5
+	loginWindow      = 15 * time.Minute
+)
+
 func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 	var req struct {
 		Username string `json:"username"`
@@ -134,8 +141,23 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Rate-limiting: block after too many recent failures
+	failures, err := s.users.CountRecentFailures(req.Username, loginWindow)
+	if err == nil && failures >= loginMaxFailures {
+		s.audlog.Log(audit.Entry{
+			EventType: audit.EventLogin,
+			Username:  req.Username,
+			IPAddress: remoteIP(r),
+			Success:   false,
+			Detail:    "rate limited",
+		})
+		writeError(w, http.StatusTooManyRequests, "too many failed login attempts, try again later")
+		return
+	}
+
 	token, user, err := s.authMgr.Login(req.Username, req.Password)
 	if err != nil {
+		_ = s.users.RecordLoginAttempt(req.Username, remoteIP(r))
 		s.audlog.Log(audit.Entry{
 			EventType: audit.EventLogin,
 			Username:  req.Username,
@@ -147,6 +169,8 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	_ = s.users.UpdateLastLogin(user.ID)
+
 	s.audlog.Log(audit.Entry{
 		EventType: audit.EventLogin,
 		Username:  user.Username,
@@ -268,6 +292,81 @@ func (s *Server) handleCreateUser(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func (s *Server) handleUpdateUser(w http.ResponseWriter, r *http.Request) {
+	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid user id")
+		return
+	}
+
+	var req struct {
+		Email    *string `json:"email"`
+		Role     *string `json:"role"`
+		Active   *bool   `json:"active"`
+		Password *string `json:"password"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	updated, err := s.users.Update(id, userstore.UpdateUserRequest{
+		Email:    req.Email,
+		Role:     req.Role,
+		Active:   req.Active,
+		Password: req.Password,
+	})
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	sess := sessionFromCtx(r.Context())
+	s.audlog.Log(audit.Entry{
+		EventType: audit.EventUserMgmt,
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Detail:    fmt.Sprintf("updated user %d", id),
+		Success:   true,
+	})
+
+	writeJSON(w, http.StatusOK, map[string]interface{}{
+		"id":       updated.ID,
+		"username": updated.Username,
+		"email":    updated.Email,
+		"role":     updated.Role,
+		"active":   updated.Active,
+	})
+}
+
+func (s *Server) handleDeleteUser(w http.ResponseWriter, r *http.Request) {
+	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid user id")
+		return
+	}
+
+	if err := s.users.DeleteSafe(id); err != nil {
+		if err.Error() == "userstore: cannot delete last admin" {
+			writeError(w, http.StatusConflict, "cannot delete the last active admin")
+			return
+		}
+		writeError(w, http.StatusNotFound, err.Error())
+		return
+	}
+
+	sess := sessionFromCtx(r.Context())
+	s.audlog.Log(audit.Entry{
+		EventType: audit.EventUserMgmt,
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Detail:    fmt.Sprintf("deleted user %d", id),
+		Success:   true,
+	})
+
+	writeJSON(w, http.StatusOK, map[string]bool{"ok": true})
+}
+
 func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 	q := r.URL.Query().Get("q")
 	fromFilter := r.URL.Query().Get("from")

internal/userstore/userstore.go

@@ -15,6 +15,8 @@ const (
 	RoleUser    = "user"
 	RoleAdmin   = "admin"
 	RoleAuditor = "auditor"
+
+	bcryptCost = 12
 )
 
 // User represents a user account in the system.
@@ -76,12 +78,20 @@ func (s *Store) initSchema(ctx context.Context) error {
 			role            VARCHAR(20)  NOT NULL CHECK (role IN ('user','auditor','admin')),
 			source          VARCHAR(20)  NOT NULL DEFAULT 'local',
 			active          BOOLEAN      NOT NULL DEFAULT true,
-			created_at    TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+			created_at      TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+			last_login_at   TIMESTAMPTZ
 		);
+		ALTER TABLE users ADD COLUMN IF NOT EXISTS last_login_at TIMESTAMPTZ;
 		CREATE TABLE IF NOT EXISTS token_blacklist (
 			jti        VARCHAR(255) PRIMARY KEY,
 			expires_at TIMESTAMPTZ  NOT NULL
 		);
+		CREATE TABLE IF NOT EXISTS login_attempts (
+			username    VARCHAR(100) NOT NULL,
+			ip          VARCHAR(45)  NOT NULL,
+			attempted_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+		);
+		CREATE INDEX IF NOT EXISTS idx_login_attempts_username_time ON login_attempts (username, attempted_at);
 	`)
 	return err
 }
@@ -94,7 +104,7 @@ func (s *Store) Close() error {
 
 // Create inserts a new local user with a bcrypt-hashed password.
 func (s *Store) Create(req CreateUserRequest) (*User, error) {
-	hash, err := bcrypt.GenerateFromPassword([]byte(req.Password), bcrypt.DefaultCost)
+	hash, err := bcrypt.GenerateFromPassword([]byte(req.Password), bcryptCost)
 	if err != nil {
 		return nil, fmt.Errorf("userstore: bcrypt: %w", err)
 	}
@@ -182,7 +192,7 @@ func (s *Store) Update(id int64, req UpdateUserRequest) (*User, error) {
 		}
 	}
 	if req.Password != nil {
-		hash, err := bcrypt.GenerateFromPassword([]byte(*req.Password), bcrypt.DefaultCost)
+		hash, err := bcrypt.GenerateFromPassword([]byte(*req.Password), bcryptCost)
 		if err != nil {
 			return nil, fmt.Errorf("userstore: bcrypt: %w", err)
 		}
@@ -257,6 +267,62 @@ func (s *Store) IsBlacklisted(jti string) (bool, error) {
 	return count > 0, err
 }
 
+// UpdateLastLogin sets last_login_at to now for the given user.
+func (s *Store) UpdateLastLogin(id int64) error {
+	ctx := context.Background()
+	_, err := s.pool.Exec(ctx, `UPDATE users SET last_login_at = NOW() WHERE id = $1`, id)
+	return err
+}
+
+// RecordLoginAttempt inserts a failed login attempt record.
+func (s *Store) RecordLoginAttempt(username, ip string) error {
+	ctx := context.Background()
+	_, err := s.pool.Exec(ctx,
+		`INSERT INTO login_attempts (username, ip, attempted_at) VALUES ($1, $2, NOW())`,
+		username, ip,
+	)
+	return err
+}
+
+// CountRecentFailures returns the number of failed attempts for username in the last window.
+func (s *Store) CountRecentFailures(username string, window time.Duration) (int, error) {
+	ctx := context.Background()
+	var count int
+	err := s.pool.QueryRow(ctx,
+		`SELECT COUNT(*) FROM login_attempts WHERE username = $1 AND attempted_at > NOW() - $2::interval`,
+		username, window.String(),
+	).Scan(&count)
+	return count, err
+}
+
+// AdminCount returns the number of active admin users.
+func (s *Store) AdminCount() (int, error) {
+	ctx := context.Background()
+	var count int
+	err := s.pool.QueryRow(ctx,
+		`SELECT COUNT(*) FROM users WHERE role = 'admin' AND active = true`,
+	).Scan(&count)
+	return count, err
+}
+
+// DeleteSafe removes a user but refuses if they are the last active admin.
+func (s *Store) DeleteSafe(id int64) error {
+	user, err := s.GetByID(id)
+	if err != nil {
+		return err
+	}
+	if user.Role == RoleAdmin {
+		count, err := s.AdminCount()
+		if err != nil {
+			return fmt.Errorf("userstore: admin count: %w", err)
+		}
+		if count <= 1 {
+			return fmt.Errorf("userstore: cannot delete last admin")
+		}
+	}
+	return s.Delete(id)
+}
+
 // CleanExpiredTokens removes blacklist entries whose expiry has passed.
 func (s *Store) CleanExpiredTokens() error {
 	ctx := context.Background()