【Feature】Plugin runtime & hooks

[gaoredu]

Is your feature request related to a problem? Please describe.

design.md defines in-process hooks (article.beforePublish, article.afterPublish, etc.), but the server only dispatches webhooks. There is no HookService, no plugins/{id}/plugin.json loader, and no way to intercept or transform data inside the CMS.

Describe the solution you'd like

• Add server/src/modules/hook/ with applyFilters / doAction
• Wire hooks into article, comment, and setting services
• Ship a sample plugin (plugins/seo-basic) using article.beforePublish

Describe alternatives you've considered

• Webhooks only — works for external sync, but cannot block publish, rewrite fields, or run spam filters in-process
• Fork server — possible for one site, not viable for a plugin ecosystem

Additional context

Priority: P0 · Blocks the plugin system · Evidence: article.service.ts → webhookService.dispatch('article.published') only