Number entries on publish, delete media, read the cover

- drafts stay unnumbered, the number is handed out when an entry goes live
- DELETE /api/v1/media/:id and a delete control in the media library
- cover_media_id in the draft read, coverMediaId and coverUrl in the archive read

tests/api/write.test.ts

@@ -65,7 +65,7 @@ describe('POST /api/v1/posts', () => {
 
 		expect(response.status).toBe(201)
 		expect(body.status).toBe('draft')
-		expect(body.number).toBe(1)
+		expect(body.number).toBeNull()
 		expect(body.slug).toBe('regel-engine')
 	})