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>

i18n/default_de.json

@@ -1285,5 +1285,28 @@
   "Session expiring": "Sitzung läuft ab",
   "Your session will expire in {countdown}. Would you like to continue?": "Ihre Sitzung läuft in {countdown} ab. Möchten Sie fortfahren?",
   "Extend session": "Sitzung verlängern",
-  "Log out": "Abmelden"
+  "Log out": "Abmelden",
+  "System Info": "Systeminfo",
+  "Health Status": "Systemstatus",
+  "Check": "Prüfung",
+  "Details": "Details",
+  "PHP version": "PHP-Version",
+  "PHP SAPI": "PHP SAPI",
+  "Active modules": "Aktive Module",
+  "No modules active.": "Keine Module aktiv.",
+  "Module": "Modul",
+  "Default": "Standard",
+  "Dependencies": "Abhängigkeiten",
+  "Permission summary": "Berechtigungsübersicht",
+  "Active permissions": "Aktive Berechtigungen",
+  "Inactive (orphaned) permissions": "Inaktive (verwaiste) Berechtigungen",
+  "Permissions by source": "Berechtigungen nach Quelle",
+  "Source": "Quelle",
+  "Count": "Anzahl",
+  "No health checks available.": "Keine Systemprüfungen verfügbar.",
+  "Warning": "Warnung",
+  "Fail": "Fehler",
+  "Environment": "Umgebung",
+  "Modules": "Module",
+  "Version": "Version"
 }