refactor: remove micro posts (微语) feature entirely

Remove the micro posts feature from the codebase including:
- Backend: API routes, handlers, and database models (MicroPost, MicroPostLike)
- Frontend: React components (Heatmap, MicroComposer, MicroList, MicroPage)
- Pages: micro.astro page and navigation links
- Documentation: API docs and user guide sections

This simplifies the application by removing a feature that is no longer needed.

BREAKING CHANGE: All micro posts related API endpoints (/api/micros) are removed.
Existing micro posts data will not be accessible after this change.

src/pages/micro.astro

@@ -1,12 +0,0 @@
----
-import BaseLayout from '../layouts/BaseLayout.astro';
-import MicroPage from '../components/react/MicroPage';
----
-
-<BaseLayout title="微语 - NovaBlog" description="分享生活点滴，记录每一个精彩瞬间">
-  <div class="py-12">
-    <div class="content-width">
-      <MicroPage client:load />
-    </div>
-  </div>
-</BaseLayout>