refactor: update handlers to use unified search endpoint

- Update SearchMediaItems handler to use SearchService
- Add autocomplete detection for field value queries (author=value, genre=value, etc.)
- Add handleFieldValuesSearch method for dropdown population
- Add sort parameter extraction with default "title ASC"
- Remove deprecated ListMediaItemsFiltered handler
- Remove deprecated /api/media-items/filtered route registration
- Update frontend.go to use SearchMediaItemsUnified instead of ListMediaItemsFiltered
- Fix parameter passing (empty filters use Valid:true with empty values, not Valid:false)
- Add SearchQuery, IsExactSearch, SearchPattern parameters for query parsing

Handler is now a thin wrapper that extracts params and delegates to service layer.

internal/router/media.go

@@ -14,7 +14,6 @@ func registerMediaRoutes(cfg *Config) {
 
 	// Media item routes (all authenticated users)
 	protected.GET("/media-items", cfg.MediaHandler.ListMediaItems)
-	protected.GET("/media-items/filtered", cfg.MediaHandler.ListMediaItemsFiltered)
 	protected.GET("/media-items/:id", cfg.MediaHandler.GetMediaItem)
 
 	// Media rating routes (all authenticated users)