Refactor search fallback and preview flows

backend/services/gemini.go

@@ -164,16 +164,21 @@ User query: ` + query,
 	}
 
 	maxImages := min(len(candidates), 10)
+	visualCount := 0
 	for idx := 0; idx < maxImages; idx++ {
 		img, mimeType, err := fetchCandidateVisualInlineData(g.Client, candidates[idx])
 		if err != nil {
 			continue
 		}
+		visualCount++
 		parts = append(parts,
 			geminiPart{"text": fmt.Sprintf("Candidate %d: title=%s source=%s link=%s", idx, candidates[idx].Title, candidates[idx].Source, candidates[idx].Link)},
 			geminiPart{"inlineData": map[string]string{"mimeType": mimeType, "data": img}},
 		)
 	}
+	if visualCount == 0 {
+		return nil, fmt.Errorf("no candidate thumbnails or preview frames could be fetched for gemini vision")
+	}
 
 	body := map[string]any{
 		"contents": []map[string]any{
@@ -248,18 +253,7 @@ User query: ` + query,
 	}
 
 	if len(recommendations) == 0 {
-		for _, candidate := range candidates[:min(8, len(candidates))] {
-			recommendations = append(recommendations, AIRecommendation{
-				Title:           candidate.Title,
-				Link:            candidate.Link,
-				Snippet:         candidate.Snippet,
-				ThumbnailURL:    candidate.ThumbnailURL,
-				PreviewVideoURL: candidate.PreviewVideoURL,
-				Source:          candidate.Source,
-				Reason:          "Gemini Vision 평가를 받지 못해 키워드 기준으로 보강된 결과입니다.",
-				Recommended:     false,
-			})
-		}
+		recommendations = BuildFallbackRecommendations(candidates, 8, "Gemini Vision 평가를 받지 못해 키워드 기준으로 보강된 결과입니다.")
 	}
 
 	return recommendations, nil

backend/services/ranker.go

@@ -1,11 +1,14 @@
 package services
 
 import (
+	"fmt"
 	"sort"
 	"strings"
 	"sync"
 )
 
+const GeminiFallbackReason = "Gemini Vision 응답이 부족해 키워드 기준으로 보강된 결과입니다."
+
 type GeminiBatchStats struct {
 	CandidateCap     int      `json:"candidateCap"`
 	Requested        int      `json:"requested"`
@@ -84,9 +87,13 @@ func GeminiCandidateLimit(total int) int {
 	return total
 }
 
-func EvaluateAllCandidatesWithGemini(service *GeminiService, query string, ranked []SearchResult) ([]AIRecommendation, GeminiBatchStats) {
+func EvaluateAllCandidatesWithGemini(service *GeminiService, query string, ranked []SearchResult) ([]AIRecommendation, GeminiBatchStats, error) {
 	const chunkSize = 8
 	const maxConcurrentBatches = 2
+	if service == nil {
+		return nil, GeminiBatchStats{}, fmt.Errorf("gemini service is not configured")
+	}
+
 	limit := GeminiCandidateLimit(len(ranked))
 	stats := GeminiBatchStats{
 		CandidateCap: limit,
@@ -106,6 +113,9 @@ func EvaluateAllCandidatesWithGemini(service *GeminiService, query string, ranke
 		batches = append(batches, ranked[start:end])
 	}
 	stats.Batches = len(batches)
+	if len(batches) == 0 {
+		return []AIRecommendation{}, stats, nil
+	}
 
 	results := make([]batchResult, len(batches))
 	var wg sync.WaitGroup
@@ -146,7 +156,41 @@ func EvaluateAllCandidatesWithGemini(service *GeminiService, query string, ranke
 		}
 	}
 	stats.RecommendedCount = len(merged)
-	return merged, stats
+
+	switch {
+	case len(merged) > 0 && stats.Failed == 0:
+		return merged, stats, nil
+	case len(merged) > 0 && stats.Failed > 0:
+		return merged, stats, fmt.Errorf("gemini vision partially failed on %d of %d batches", stats.Failed, stats.Batches)
+	case stats.Failed == stats.Batches:
+		if len(stats.Errors) > 0 {
+			return nil, stats, fmt.Errorf("gemini vision failed for all batches: %s", strings.Join(stats.Errors, "; "))
+		}
+		return nil, stats, fmt.Errorf("gemini vision failed for all batches")
+	default:
+		return nil, stats, fmt.Errorf("gemini vision returned no candidate evaluations")
+	}
+}
+
+func BuildFallbackRecommendations(ranked []SearchResult, limit int, reason string) []AIRecommendation {
+	if strings.TrimSpace(reason) == "" {
+		reason = GeminiFallbackReason
+	}
+
+	fallback := make([]AIRecommendation, 0, min(limit, len(ranked)))
+	for _, item := range ranked[:min(limit, len(ranked))] {
+		fallback = append(fallback, AIRecommendation{
+			Title:           item.Title,
+			Link:            item.Link,
+			Snippet:         item.Snippet,
+			ThumbnailURL:    item.ThumbnailURL,
+			PreviewVideoURL: item.PreviewVideoURL,
+			Source:          item.Source,
+			Reason:          reason,
+			Recommended:     false,
+		})
+	}
+	return fallback
 }
 
 func MergeRecommendations(recommended []AIRecommendation, ranked []SearchResult, limit int) []AIRecommendation {
@@ -184,7 +228,7 @@ func MergeRecommendations(recommended []AIRecommendation, ranked []SearchResult,
 			ThumbnailURL:    item.ThumbnailURL,
 			PreviewVideoURL: item.PreviewVideoURL,
 			Source:          item.Source,
-			Reason:          "Gemini Vision 응답이 부족해 키워드 기준으로 보강된 결과입니다.",
+			Reason:          GeminiFallbackReason,
 			Recommended:     false,
 		})
 	}