Address two residual bypasses identified in review:
1. Add _plugin_api_runtime_gate middleware that checks plugins.enabled/
plugins.disabled on every request to /api/plugins/{name}/... routes.
Previously, disabling a plugin at runtime had no effect on its already-
mounted API routes until a restart.
2. Extend serve_plugin_asset to check plugins.disabled for bundled plugins.
Previously, only user plugins were gated — a bundled plugin in
plugins.disabled would still serve assets from the unauthenticated
/dashboard-plugins/{name}/... endpoint.
Both fixes ensure the enabled/disabled policy is evaluated live at request
time, not just at startup.
Adds regression tests covering:
- Middleware blocks disabled user plugin API routes (404)
- Middleware blocks user plugin removed from enabled set (404)
- Middleware passes enabled user plugin API routes
- Middleware blocks disabled bundled plugin API routes (404)
- Bundled plugin assets return 404 when disabled
- Bundled plugin assets served normally when not disabled
- User plugin asset gating still works correctly
