Log fallback-scan truncation and drop dead blank-assignment block

- scanDBCandidates now warns when the fallback scan hits scanRowCap, so a
  silently truncated result set is observable instead of looking complete.
- Remove the pointless `var ( _ = dbCandidateFloor; _ = dbCandidateCap )`
  block: both constants are used by DBCandidateLimit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: tae2089

internal/retrieval/retrieval.go

@@ -41,8 +41,3 @@ const (
 	// this behave identically to an uncapped scan; larger ones are truncated in stable order.
 	scanRowCap = 5000
 )
-
-var (
-	_ = dbCandidateFloor
-	_ = dbCandidateCap
-)

internal/retrieval/service.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"log/slog"
 	"os"
 	"sort"
 	"strings"
@@ -126,6 +127,11 @@ func (s *Service) scanDBCandidates(ctx context.Context, namespace, query string)
 		Find(&nodes).Error; err != nil {
 		return nil, fmt.Errorf("retrieve docs DB candidates: %w", err)
 	}
+	if len(nodes) == scanRowCap {
+		// The scan hit its ceiling: matching nodes sorting after the cap are not considered.
+		slog.WarnContext(ctx, "retrieve_docs fallback scan truncated at cap; some matches may be omitted",
+			"namespace", namespace, "cap", scanRowCap)
+	}
 	if len(nodes) == 0 {
 		return nil, nil
 	}