diag(matcher): debug-log provider search query + per-provider result counts

Adds DEBUG logs in the ModeInitialMatch search path: each provider's result
count for the query, and the assembled raw/candidate totals. Lets us see when a
provider search returns zero ('no metadata found') vs a scoring/tie issue.
Zero behavior change.

internal/metadata/service.go

@@ -886,6 +886,12 @@ func (s *MetadataService) processInternal(ctx context.Context, req ProcessReques
 					"provider", p.Slug(), "error", err)
 				continue
 			}
+			slog.Debug("metadata: provider search result",
+				"provider", p.Slug(),
+				"query_title", searchQuery.Title,
+				"query_year", searchQuery.Year,
+				"result_count", len(results),
+			)
 			for _, result := range results {
 				if searchResultConflictsWithTrustedIDs(accumulatedIDs, result.ProviderIDs) {
 					slog.Warn("metadata: skipping conflicting search result",
@@ -902,6 +908,12 @@ func (s *MetadataService) processInternal(ctx context.Context, req ProcessReques
 		}
 
 		candidates := NormalizeCandidates(allResults, contentType)
+		slog.Debug("metadata: search candidates assembled",
+			"query_title", searchQuery.Title,
+			"query_year", searchQuery.Year,
+			"raw_results", len(allResults),
+			"candidates", len(candidates),
+		)
 		providerPriority := make([]string, 0, len(itemChain))
 		for _, p := range itemChain {
 			providerPriority = append(providerPriority, p.Slug())