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/purge_deleted_annotation.md

@@ -0,0 +1,36 @@
+---
+method: DELETE
+path: /api/media-items/:id/annotations/:annotationId
+auth: jwt
+area: media-items
+---
+<!-- source: internal/handlers/annotation_history.go:101 MediaHandler.PurgeDeletedAnnotation -->
+
+# Purge Deleted Annotation
+
+**Permanent** — removes a tombstoned annotation from history entirely. Kind
+comes from the required `annotation_type` query parameter
+(`highlight` | `note` | `bookmark`; no body fallback on DELETE).
+
+## Example request
+
+```text
+DELETE /api/media-items/550e8400-.../annotations/440e8400-...?annotation_type=highlight
+```
+
+## Example response
+
+```json example-response
+{
+  "purged": true
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "annotation_type must be highlight, note, or bookmark"}` | missing/invalid kind |
+| 404 | `{"error": "deleted annotation not found"}` | nothing tombstoned under that id |
+| 500 | `{"error": "failed to purge annotation"}` | unexpected server error |
+| 503 | `{"error": "annotation service unavailable"}` | service not constructed |