feat(api): add POST /api/media-items/:id/rescan endpoint

Admin-only endpoint that re-extracts metadata and cover art for a single
book from the file on disk via MediaScanner.RescanMediaItem and returns
the updated media item. Normal library scans skip unchanged files, so
this gives a targeted way to backfill covers for previously imported
books. Includes a Bruno request alongside the existing media-items
collection.

bruno/media-items/Rescan Media Item.yml

@@ -0,0 +1,35 @@
+info:
+  name: Rescan Media Item
+  type: http
+  seq: 9
+
+http:
+  method: POST
+  url: "{{base_url}}/api/media-items/{{media_item_id}}/rescan"
+  headers:
+    - name: ""
+      value: application/json
+  auth: inherit
+
+runtime:
+  scripts:
+    - type: tests
+      code: |-
+        test_rescan_media_item_success(status, headers, body) {
+            if (status !== 200) {
+              throw new Error("Expected status 200, got " + status);
+
+settings:
+  encodeUrl: true
+  timeout: 0
+  followRedirects: true
+  maxRedirects: 5
+
+docs: |-
+  ## Rescan Media Item
+
+    Re-extracts metadata and cover art for a single media item from the file on disk.
+
+    **Method:** POST
+
+    **Endpoint:** /api/media-items/{id}/rescan