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.

internal/router/media.go

@@ -60,6 +60,7 @@ func registerMediaRoutes(cfg *Config) {
 	// Admin-only media routes
 	admin.POST("/media-items", cfg.MediaHandler.CreateMediaItem)
 	admin.PUT("/media-items/:id", cfg.MediaHandler.UpdateMediaItem)
+	admin.POST("/media-items/:id/rescan", cfg.MediaHandler.RescanMediaItem)
 	admin.DELETE("/media-items/:id", cfg.MediaHandler.DeleteMediaItem)
 
 	// Shelf management (protected)