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/components/SendLogs.vue

@@ -0,0 +1,42 @@
+<template>
+  <div>
+    <div class="log-toolbar">
+      <span class="kpi-label">LAST {{ logs.length }} SENDS</span>
+      <button class="btn btn-ghost btn-sm" :disabled="loading" @click="refresh">⟳ Refresh</button>
+    </div>
+
+    <p v-if="error" class="err">{{ error }}</p>
+    <p v-else-if="!loading && !logs.length" class="empty-log">No send records yet — trigger a webhook to see results.</p>
+
+    <div class="log-list">
+      <article v-for="l in logs" :key="l.ts + l.routeId" class="log-entry" :class="{ fail: !l.ok }">
+        <div class="log-head">
+          <span class="dot" :class="l.ok ? 'ok' : 'bad'"></span>
+          <span class="log-route">{{ l.routeId }}</span>
+          <span class="log-event">{{ l.event }}</span>
+          <span class="log-time">{{ fmtTime(l.ts) }}</span>
+        </div>
+        <div class="log-meta">
+          <span v-if="l.repo"><b>REPO</b><code>{{ l.repo }}</code></span>
+          <span><b>TARGET</b><code>{{ l.target }}</code></span>
+        </div>
+        <div v-if="!l.ok && l.error" class="log-error">{{ l.error }}</div>
+      </article>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import type { SendRecord } from "~/types";
+
+const props = defineProps<{ logs: SendRecord[]; loading: boolean }>();
+const emit = defineEmits<{ (e: "refresh"): void }>();
+
+function fmtTime(ts: number): string {
+  return new Date(ts).toLocaleString();
+}
+
+function refresh(): void {
+  emit("refresh");
+}
+</script>