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.

server/internal/database/database.go

@@ -54,8 +54,6 @@ func autoMigrate() error {
 		&models.Like{},
 		&models.LikeCount{},
 		&models.PostMeta{},
-		&models.MicroPost{},
-		&models.MicroPostLike{},
 	)
 }