feat(admin): Discounts and Website Settings sections

- /admin/discounts: list + create discount codes (kind, value, max uses, expiry)
  via /v1/discounts (backend has no edit/delete API yet)
- /admin/settings: key/value site settings with upsert + secret flag. The value
  column is jsonb, so values are JSON-encoded on save / decoded for display
- nav links + fa/en labels

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

messages/en.json

@@ -321,7 +321,9 @@
       "users": "Users",
       "plans": "Plans",
       "templates": "Templates",
-      "media": "Media"
+      "media": "Media",
+      "discounts": "Discounts",
+      "siteSettings": "Settings"
     },
     "appAdminNodesPage": {
       "title": "Render Nodes",