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>

db/updates/2026-03-22-system-info-permission.sql

@@ -0,0 +1,14 @@
+-- Add system_info.view permission for existing installs.
+-- Idempotent: ON DUPLICATE KEY prevents re-insert.
+
+INSERT INTO `permissions` (`key`, `description`, `active`, `is_system`)
+VALUES ('system_info.view', 'Can view system info and health status', 1, 1)
+ON DUPLICATE KEY UPDATE `description` = VALUES(`description`);
+
+-- Assign to Admin role(s) for existing installs.
+INSERT INTO `role_permissions` (`role_id`, `permission_id`, `created`)
+SELECT r.id, p.id, NOW()
+FROM roles r
+JOIN permissions p ON p.`key` = 'system_info.view'
+WHERE r.description IN ('Admin', 'Administrator') OR r.id = 1
+ON DUPLICATE KEY UPDATE role_id = role_id;