feat: add multi-administrator support with RBAC and fix backup/restore for S3

## Multi-Administrator System
- Add role-based access control (RBAC) with predefined roles (Super Admin, Admin, Editor, Viewer)
- Add granular permissions system for all admin operations
- Add admin user management page with invite functionality
- Add email invitation system for new administrators
- Add permission middleware protecting all admin routes
- Add PermissionGate component for frontend permission checks
- Track event creator (created_by) for audit purposes

## Backup & Restore Fixes
- Fix S3 backup: endpoint URL handling, manifest loading, field name compatibility
- Fix S3 restore: add list-backups endpoint, transform S3 config from frontend format
- Fix PostgreSQL compatibility: add .returning('id') for insert operations
- Fix disk space check: use df command, handle unknown space gracefully
- Fix dry-run validation to not block on warnings
- Fix req.user → req.admin in restore routes

## Database Migrations
- 054: Add roles table with predefined roles
- 055: Add permissions table
- 056: Add role_permissions junction table
- 057: Add role_id to admin_users
- 058: Add admin_invitations table
- 059: Add admin email templates
- 060: Add created_by to events table

## Other Improvements
- Update .gitignore to exclude planning docs and local backup directory
- Remove SQLite database file from tracking
- Add i18n translations for user management (EN/DE)

backend/src/routes/adminSystem.js

@@ -1,6 +1,7 @@
 const express = require('express');
 const { db, withRetry } = require('../database/db');
 const { adminAuth } = require('../middleware/auth');
+const { requirePermission } = require('../middleware/permissions');
 const fs = require('fs').promises;
 const path = require('path');
 const os = require('os');
@@ -9,7 +10,7 @@ const logger = require('../utils/logger');
 const router = express.Router();
 
 // Get system version
-router.get('/version', adminAuth, async (req, res) => {
+router.get('/version', adminAuth, requirePermission('settings.view'), async (req, res) => {
   try {
     // Read backend version from package.json
     let backendVersion = '1.0.0';
@@ -35,7 +36,7 @@ router.get('/version', adminAuth, async (req, res) => {
 });
 
 // Get comprehensive system status
-router.get('/status', adminAuth, async (req, res) => {
+router.get('/status', adminAuth, requirePermission('settings.view'), async (req, res) => {
   try {
     // Database size - check if PostgreSQL or SQLite
     let dbSize = 0;
@@ -170,7 +171,7 @@ router.get('/status', adminAuth, async (req, res) => {
 });
 
 // Get database statistics
-router.get('/database', adminAuth, async (req, res) => {
+router.get('/database', adminAuth, requirePermission('settings.view'), async (req, res) => {
   try {
     // Get table info
     const tables = [