security: Zufallspasswörter beim Erststart, kryptographisch sichere JTI-Generierung

- seedDefaultUsers: generiert kryptographisch zufällige Passwörter (crypto/rand)
  statt hartkodiertes "archivmailrockz" — Passwörter werden einmalig im Terminal
  angezeigt und können danach nicht wiederhergestellt werden
- generateJTI: verwendet crypto/rand (16 Byte, hex) statt time.UnixNano XOR deadbeef

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

cmd/archivmail/main.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"context"
+	"crypto/rand"
+	"encoding/hex"
 	"flag"
 	"fmt"
 	"log/slog"
@@ -321,6 +323,8 @@ func runIntegrityCheck(ctx context.Context, store *storage.Store, logger *slog.L
 }
 
 // seedDefaultUsers creates default admin and auditor accounts if no users exist yet.
+// Passwords are randomly generated and printed once to stdout — there is no way to
+// recover them afterwards; they must be changed immediately after the first login.
 func seedDefaultUsers(users *userstore.Store, logger *slog.Logger) error {
 	all, err := users.List("")
 	if err != nil {
@@ -329,16 +333,46 @@ func seedDefaultUsers(users *userstore.Store, logger *slog.Logger) error {
 	if len(all) > 0 {
 		return nil // already seeded
 	}
+
+	adminPw, err := randomPassword()
+	if err != nil {
+		return fmt.Errorf("generate admin password: %w", err)
+	}
+	auditorPw, err := randomPassword()
+	if err != nil {
+		return fmt.Errorf("generate auditor password: %w", err)
+	}
+
 	defaults := []userstore.CreateUserRequest{
-		{Username: "admin", Email: "admin@archivmail.local", Password: "archivmailrockz", Role: userstore.RoleAdmin},
-		{Username: "auditor", Email: "auditor@archivmail.local", Password: "archivmailrockz", Role: userstore.RoleAuditor},
+		{Username: "admin", Email: "admin@archivmail.local", Password: adminPw, Role: userstore.RoleAdmin},
+		{Username: "auditor", Email: "auditor@archivmail.local", Password: auditorPw, Role: userstore.RoleAuditor},
 	}
 	for _, req := range defaults {
 		if _, err := users.Create(req); err != nil {
 			return fmt.Errorf("create default user %s: %w", req.Username, err)
 		}
-		logger.Info("created default user", "username", req.Username, "role", req.Role)
 	}
-	logger.Warn("default users created — change passwords immediately!", "admin", "admin", "auditor", "auditor")
+
+	// Print credentials prominently — this is the only time they are visible.
+	fmt.Println()
+	fmt.Println("╔══════════════════════════════════════════════════════════════╗")
+	fmt.Println("║         ARCHIVMAIL — ERSTMALIGE EINRICHTUNG                 ║")
+	fmt.Println("║  Initiale Zugangsdaten (NUR EINMAL ANGEZEIGT):              ║")
+	fmt.Printf( "║  admin   : %-50s ║\n", adminPw)
+	fmt.Printf( "║  auditor : %-50s ║\n", auditorPw)
+	fmt.Println("║  Passwörter sofort nach dem ersten Login ändern!            ║")
+	fmt.Println("╚══════════════════════════════════════════════════════════════╝")
+	fmt.Println()
+
+	logger.Warn("default users created — change passwords immediately!")
 	return nil
 }
+
+// randomPassword generates a cryptographically random 16-byte hex password.
+func randomPassword() (string, error) {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(b), nil
+}