feat: use xpier media library for uploads and markdown embeds

Store media:ID in post bodies instead of CDN URLs, and keep sablog attach tokens as a fallback.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -79,6 +79,10 @@ WORKERMAN_TIMEOUT=60
 
 # ---- 附件 S3 兼容存储（R2 / COS / OSS / MinIO），不配置则回退本地 public ----
 UPLOAD_DISK=uploads
+# New uploads/covers use xpier media library (keep sablog Spatie attachments as fallback)
+MEDIA_DISK=public
+MEDIA_DIRECTORY=media
+MEDIA_VISIBILITY=public
 S3_ACCESS_KEY=
 S3_SECRET_KEY=
 S3_REGION=auto