feat(plugin): admin-configurable marketplace URL as the install default

The Settings install command had a <your-scribe-repo> placeholder — not
copyable. Add an instance-global 'plugin_marketplace_url' setting (admin sets
it to the app's own repo) that every user's MCP Access reads, so the
/plugin marketplace add command is copyable out of the box. Keeps it universal
(each deployment configures its own repo) rather than hardcoding one.

- services/settings.get_admin_setting(key): admin-scoped global read.
- routes/plugin: GET /api/plugin/marketplace-url (any user) + PUT (admin).
- SettingsView: Admin → 'Plugin marketplace' field to set it; MCP Access
  marketplace field falls back to the configured value.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -144,8 +144,16 @@ const claudeCodeCommand = computed(() => {
 const _MKT_KEY = 'plugin_marketplace_url';
 const pluginMarketplaceUrl = ref<string>(localStorage.getItem(_MKT_KEY) || '');
 watch(pluginMarketplaceUrl, (v) => localStorage.setItem(_MKT_KEY, (v || '').trim()));
+// Instance default, set by an admin (Admin tab) and loaded on mount. Used when
+// the per-browser field is blank so the install command is copyable out of the box.
+const serverMarketplaceUrl = ref('');
+const adminMarketplaceUrl = ref('');
+const savingMarketplaceUrl = ref(false);
+const marketplaceUrlSaved = ref(false);
 const pluginInstallCommands = computed(() => {
-  const mkt = (pluginMarketplaceUrl.value || '').trim() || '<your-scribe-repo>.git';
+  const mkt = (pluginMarketplaceUrl.value || '').trim()
+    || serverMarketplaceUrl.value
+    || '<your-scribe-repo>.git';
   return `/plugin marketplace add ${mkt}\n/plugin install scribe@scribe-plugin`;
 });
 
@@ -429,6 +437,17 @@ onMounted(async () => {
     searxngConfigured.value = false;
   }
 
+  // Plugin marketplace URL (instance default; readable by all users so the
+  // install command in MCP Access is copyable).
+  try {
+    const mk = await apiGet<{ marketplace_url: string }>("/api/plugin/marketplace-url");
+    serverMarketplaceUrl.value = mk.marketplace_url || "";
+    adminMarketplaceUrl.value = mk.marketplace_url || "";
+    if (!pluginMarketplaceUrl.value) pluginMarketplaceUrl.value = mk.marketplace_url || "";
+  } catch {
+    // not configured yet
+  }
+
   // Load admin settings
   if (authStore.isAdmin) {
     try {
@@ -658,6 +677,26 @@ async function saveBaseUrl() {
   }
 }
 
+async function saveMarketplaceUrl() {
+  savingMarketplaceUrl.value = true;
+  marketplaceUrlSaved.value = false;
+  try {
+    const url = adminMarketplaceUrl.value.trim();
+    await apiPut("/api/plugin/marketplace-url", { marketplace_url: url });
+    serverMarketplaceUrl.value = url;
+    // Reflect the new instance default in the copyable field unless the user
+    // set their own per-browser override.
+    if (!localStorage.getItem(_MKT_KEY)) pluginMarketplaceUrl.value = url;
+    marketplaceUrlSaved.value = true;
+    setTimeout(() => (marketplaceUrlSaved.value = false), 2000);
+  } catch (e) {
+    const body = (e as { body?: { error?: string } }).body;
+    toastStore.show(body?.error || "Failed to save marketplace URL", "error");
+  } finally {
+    savingMarketplaceUrl.value = false;
+  }
+}
+
 async function handleRestoreFile(event: Event) {
   const file = (event.target as HTMLInputElement).files?.[0];
   if (!file) return;
@@ -1663,6 +1702,31 @@ function formatUserDate(iso: string): string {
         </div>
       </section>
 
+      <section class="settings-section full-width">
+        <h2>Plugin marketplace</h2>
+        <p class="section-desc">
+          Git URL of the repo that ships this Scribe plugin (usually this app's own repo).
+          Shown to every user in <strong>MCP Access</strong> so the install command is
+          copyable. Example: https://git.example.com/you/Scribe.git
+        </p>
+        <div class="field url-field">
+          <label for="marketplace-url">Marketplace git URL</label>
+          <input
+            id="marketplace-url"
+            v-model="adminMarketplaceUrl"
+            type="url"
+            placeholder="https://git.example.com/you/Scribe.git"
+            class="input"
+          />
+        </div>
+        <div class="actions">
+          <button class="btn-save" @click="saveMarketplaceUrl" :disabled="savingMarketplaceUrl">
+            {{ savingMarketplaceUrl ? "Saving..." : "Save" }}
+          </button>
+          <span v-if="marketplaceUrlSaved" class="saved-msg">Saved!</span>
+        </div>
+      </section>
+
       <section class="settings-section full-width">
         <h2>Email / SMTP</h2>
         <p class="section-desc">Configure SMTP to enable email notifications for all users.</p>