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/Repository/System/SystemHealthRepositoryInterface.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace MintyPHP\Repository\System;
+
+interface SystemHealthRepositoryInterface
+{
+    public function checkDatabaseConnectivity(): bool;
+
+    /**
+     * @param list<string> $requiredTables
+     * @return list<string>
+     */
+    public function listPresentTables(array $requiredTables): array;
+
+    /**
+     * @return array{last_heartbeat_at: string, last_result: string, last_error_code: string}|null
+     */
+    public function getSchedulerStatus(): ?array;
+
+    /**
+     * @param list<string> $keys
+     * @return list<string>
+     */
+    public function listActivePermissionKeys(array $keys): array;
+
+    public function countAdminUsers(): int;
+
+    public function countActivePermissions(): int;
+
+    public function countInactivePermissions(): int;
+
+    /**
+     * @return list<string>
+     */
+    public function listAllActivePermissionKeys(): array;
+}