docs(media-items): Work Order 3 batch 3a — reader-adjacent CRUD (11 endpoints)

New frontmatter'd docs with real shapes (sqlc models pinned directly):
- bookmarks: get/create/update/delete (sync-aware rows: dedup_key,
  last_modified_*, tombstones; delete is 204)
- review: upsert (empty string DELETES the review) / get (200 with
  {review:null} when absent) / delete
- get single note + highlight (full sqlc row shapes)
- annotation history: restore (POST, kind via query or body) and purge
  (DELETE, kind via required query; permanent)

Gate hardening forced by this batch:
- required-field walk now guards non-struct Request types (RawMessage pins)
- update_media_bookmark needed a full example block (no cross-reference
  shortcuts — the gate checks every doc standalone)

docs/developer/api/media-items/delete_media_bookmark.md

@@ -0,0 +1,20 @@
+---
+method: DELETE
+path: /api/media-items/:id/bookmarks/:bookmarkId
+auth: jwt
+area: media-items
+---
+<!-- source: internal/handlers/media.go:2304 MediaHandler.DeleteMediaBookmark -->
+
+# Delete Media Bookmark
+
+Tombstones a bookmark (sync-aware delete, source "web"). Success is
+**204 No Content** — no body.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid bookmark id"}` / `{"error": "invalid user"}` | malformed |
+| 404 | `{"error": "bookmark not found"}` | no such bookmark |
+| 500 | `{"error": "..."}` | unexpected server error |