Add an admin WebUI (Nuxt static SPA in admin/, served via the ASSETS
binding) for managing routes and viewing send logs. Access is gated by
GitHub OAuth plus an ADMIN_USER_IDS whitelist with cookie sessions
(admin-session.ts). Expose routes/logs CRUD API (admin-routes.ts), add a
discord-link mapping in token-store.ts, and mount admin routes with an
SPA assets fallback in the server.
Gateway is now an opt-in feature (DISCORD_GATEWAY_ENABLED); when disabled,
messages are delivered via the Discord REST API (discord-rest.ts) instead of
the Durable Object. Record per-route delivery results (send-log.ts) for the
WebUI. Configure assets binding, DO sqlite migration and cron trigger.
Remove hardcoded DEFAULT_ROUTES and Config.discord.defaultChannelId so
forwarding is driven entirely by user-defined routes (empty KV = no
forwarding). Add ADMIN_USER_IDS, DISCORD_GATEWAY_ENABLED and ASSETS to Env.
