feat: add read-only System Info admin page with health checks and module inventory

New page at /admin/system-info with three tabs:
- Overview: PHP version, SAPI, environment, 6 health checks (DB, schema,
  storage, RBAC baseline, admin role, scheduler heartbeat)
- Modules: table of active modules with version, dependencies, permission count
- Permissions: active/inactive counts and per-source breakdown

Gated behind new system_info.view permission assigned to Admin role.
No mutations — purely diagnostic/observability.

Includes SystemHealthService, SystemInfoService, SystemHealthRepository
with interface, DI registration, i18n keys (de+en), idempotent DB update
script, and 17 new PHPUnit tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/Service/Access/PermissionService.php

@@ -82,6 +82,7 @@ class PermissionService
     public const SYSTEM_AUDIT_VIEW = 'system_audit.view';
     public const SYSTEM_AUDIT_PURGE = 'system_audit.purge';
     public const STATS_VIEW = 'stats.view';
+    public const SYSTEM_INFO_VIEW = 'system_info.view';
     public const API_TOKENS_MANAGE = 'api_tokens.manage';
 
     public function userHas(int $userId, string $permissionKey): bool