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>

docs/media.md

@@ -0,0 +1,32 @@
+# Media library (xpier)
+
+New uploads, covers, and in-body images/files use `xpier/filament-media-library`. Sablog Spatie media / `attachments` rows stay for import and legacy shortcodes.
+
+## Markdown body (do not paste CDN URLs)
+
+The editor writes a **stable reference**. The public URL is resolved only when the page is rendered.
+
+| Format   | Image              | File                    |
+| -------- | ------------------ | ----------------------- |
+| Markdown | `![alt](media:12)` | `[notes.pdf](media:12)` |
+| HTML     | `[media=12]`       | `[media=12]`            |
+
+Admin flow: pick a file in **Insert from media library**, then click **Insert into body**. That appends the snippet above. Do not copy object-storage / CDN addresses into the article.
+
+Legacy sablog tokens are unchanged: `[attach=ID]`, `{{attach:ID}}`, `![alt](attach:ID)`.
+
+## Admin
+
+- Library: `/admin/media-library`
+- Folders: `/admin/media-folders` (defaults: `general`, `articles`, `banners`)
+- Cover: `MediaPicker` bound to `coverMedia` / `cover_media_id`
+
+## Config
+
+```env
+MEDIA_DISK=public
+MEDIA_DIRECTORY=media
+MEDIA_VISIBILITY=public
+```
+
+Run `php artisan storage:link` when using the `public` disk locally.