feat(admin): GitHub OAuth login, admin whitelist and Nuxt WebUI

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.

admin/types.ts

@@ -0,0 +1,43 @@
+export interface Filter {
+  type: "event" | "repo" | "actor" | "action" | "branch" | "keyword";
+  match: string | string[];
+  exclude?: boolean;
+}
+
+export interface Route {
+  id: string;
+  name: string;
+  enabled: boolean;
+  filters: Filter[];
+  target: {
+    channelId: string;
+    threadId?: string;
+  };
+  lang?: string;
+}
+
+export const FILTER_TYPES = ["event", "repo", "actor", "action", "branch", "keyword"] as const;
+
+export const FILTER_LABELS: Record<string, string> = {
+  event: "Event",
+  repo: "Repo",
+  actor: "Actor",
+  action: "Action",
+  branch: "Branch",
+  keyword: "Keyword",
+};
+
+export function fmtMatch(match: string | string[]): string {
+  if (Array.isArray(match)) return match.join(", ");
+  return String(match ?? "");
+}
+
+export interface SendRecord {
+  ts: number;
+  routeId: string;
+  event: string;
+  repo?: string;
+  target: string;
+  ok: boolean;
+  error?: string;
+}