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/get_media_highlight.md

@@ -0,0 +1,53 @@
+---
+method: GET
+path: /api/media-items/:id/highlights/:highlightId
+auth: jwt
+area: media-items
+---
+<!-- source: internal/handlers/media.go:2078 MediaHandler.GetMediaHighlight -->
+
+# Get Media Highlight
+
+Returns one highlight by id.
+
+## Example response
+
+```json example-response
+{
+  "id": "440e8400-e29b-41d4-a716-446655440000",
+  "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+  "user_id": "92413c3a-e1b8-4fff-b3f9-95c74478d19a",
+  "selection_text": "It was a bright cold day in April...",
+  "start_position": "kobo:3.1",
+  "end_position": "kobo:3.4",
+  "color": "yellow",
+  "note_id": null,
+  "created_at": "2026-10-24T13:57:01Z",
+  "updated_at": "2026-10-24T13:57:01Z",
+  "percentage_start": 41.2,
+  "percentage_end": 41.8,
+  "character_start": null,
+  "character_end": null,
+  "epubcfi_start": null,
+  "epubcfi_end": null,
+  "chapter_reference": 3,
+  "paragraph_start": null,
+  "paragraph_end": null,
+  "panel_number": null,
+  "device_sync_data": null,
+  "dedup_key": "kobo:3.1-3.4",
+  "last_modified_at": "2026-10-24T13:57:01Z",
+  "last_modified_source": "web",
+  "note_text": null,
+  "deleted": false,
+  "deleted_at": null
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid highlight id"}` | malformed id |
+| 404 | `{"error": "highlight not found"}` | no such highlight |
+| 500 | `{"error": "..."}` | unexpected server error |