cache: add QueryHash and CachedEngineResponse type

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/cache/cache.go

@@ -208,3 +208,25 @@ func Key(req contracts.SearchRequest) string {
 
 	return hex.EncodeToString(h.Sum(nil))[:32]
 }
+
+// QueryHash computes a deterministic hash from shared request parameters
+// (query, pageno, safesearch, language, time_range) for use as a cache key suffix.
+// The hash is a truncated SHA-256 (16 hex chars).
+func QueryHash(query string, pageno int, safesearch int, language, timeRange string) string {
+	h := sha256.New()
+	fmt.Fprintf(h, "q=%s|", query)
+	fmt.Fprintf(h, "pageno=%d|", pageno)
+	fmt.Fprintf(h, "safesearch=%d|", safesearch)
+	fmt.Fprintf(h, "lang=%s|", language)
+	if timeRange != "" {
+		fmt.Fprintf(h, "tr=%s|", timeRange)
+	}
+	return hex.EncodeToString(h.Sum(nil))[:16]
+}
+
+// CachedEngineResponse wraps an engine's cached response with metadata.
+type CachedEngineResponse struct {
+	Engine   string
+	Response []byte
+	StoredAt time.Time
+}