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/init/init.sql

@@ -1052,6 +1052,7 @@ VALUES
   ('system_audit.purge', 'Can purge system audit logs', 1, 1),
   ('api_tokens.manage', 'Can manage user API tokens', 1, 1),
   ('stats.view', 'Can view statistics', 1, 1),
+  ('system_info.view', 'Can view system info and health status', 1, 1),
   ('roles.assign_all', 'Can assign all roles (bypass assignable-roles check)', 1, 1),
   ('notifications.view', 'Can view and manage own notifications', 1, 1)
 ON DUPLICATE KEY UPDATE
@@ -1153,7 +1154,7 @@ JOIN permissions p ON p.`key` IN (
   'api_docs.view', 'docs.view',
   'api_tokens.manage',
   'mail_log.view', 'api_audit.view', 'system_audit.view', 'system_audit.purge',
-  'stats.view',
+  'stats.view', 'system_info.view',
   'roles.assign_all'
 )
 WHERE r.description IN ('Admin', 'Administrator') OR r.id = 1