feat(admin): surface configuration problems in the admin panel

Add GET /api/admin/health powered by a config health service that
detects common deployment mistakes: missing or read-only data volume,
broken tariffs/locale-override/guides JSON files, payment providers
enabled without credentials, webhook providers without
WEBHOOK_BASE_URL, no enabled payment methods, missing or non-https
mini app URL, missing Redis, partially configured SMTP, untrusted
reverse proxy, invalid bot token, missing/mismatched/failing Telegram
webhook and unreachable Remnawave panel. Network checks (Telegram,
panel) are cached for two minutes; ?refresh=1 forces a re-check.

The admin UI shows the alerts as a banner on the dashboard with
per-section navigation chips and a manual re-check button, and as a
filtered banner inside each affected section. Alerts are localized
via admin_health_* keys with built-in Russian fallbacks.

backend/bot/app/web/admin_api.py

@@ -9,6 +9,7 @@ from bot.app.web.admin_api_impl import (
     backups as _backups,
     broadcast as _broadcast,
     common as _common,
+    health as _health,
     logs as _logs,
     panel as _panel,
     payments as _payments,
@@ -28,6 +29,7 @@ _MODULES = (
     _runtime,
     _auth,
     _common,
+    _health,
     _stats,
     _users,
     _payments,