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>

src/scribe/services/settings.py

@@ -4,10 +4,28 @@ from sqlalchemy import delete as sa_delete, select
 
 from scribe.models import async_session
 from scribe.models.setting import Setting
+from scribe.models.user import User
 
 logger = logging.getLogger(__name__)
 
 
+async def get_admin_setting(key: str, default: str = "") -> str:
+    """Read an instance-global setting (one stored on an admin account).
+
+    Used for settings that aren't per-user — e.g. the plugin marketplace URL
+    shown to everyone in Settings. Mirrors the admin-scoped lookup used for
+    the application base URL.
+    """
+    async with async_session() as session:
+        result = await session.execute(
+            select(Setting)
+            .join(User, Setting.user_id == User.id)
+            .where(User.role == "admin", Setting.key == key)
+        )
+        setting = result.scalars().first()
+        return setting.value if setting and setting.value else default
+
+
 async def get_setting(user_id: int, key: str, default: str = "") -> str:
     async with async_session() as session:
         result = await session.execute(