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>

Fetures.md

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+# LaraLog Features
+
+Modern blog migrated from SaBlog-X: Filament admin, themes, plugins, sablog import, S3 attachments, Workerman AI queue.
+
+## Local development
+
+- Site: http://laralog.test
+- Admin: http://laralog.test/admin
+- Account: `admin@laralog.test` / `password`
+- Database: MySQL `laralog` (Herd, root / empty password)
+- Remote: `git@gitea.neatcn.com:gouki/laralog.git`
+
+See `README.md` and `docs/` for install, import, plugins, and themes.
+
+## Media library (xpier)
+
+- Package: `xpier/filament-media-library` (admin Media Library + MediaPicker).
+- **Markdown body must store `![alt](media:ID)` or `[name](media:ID)`**, never a CDN/object-storage URL. HTML uses `[media=ID]`.
+- Insert flow: pick a file → **Insert into body**. Cover uses `cover_media_id`.
+- Legacy sablog `[attach=ID]` / `{{attach:ID}}` still render from Spatie media.
+- Details: `docs/media.md`.