fix(search): add json tags to FieldValue struct for correct API response

FieldValue struct had no json tags, so Go marshaled fields as uppercase
(Value, Count, Score) but frontend expected lowercase (value, count).
This caused all autocomplete dropdowns (tags, author, series, language)
to silently fail — tagSuggestions[].value was undefined, crashing
toLowerCase() calls and producing empty dropdowns.

internal/services/search.go

@@ -111,9 +111,9 @@ type FieldSearchParams struct {
 
 // FieldValue represents a single field value with metadata
 type FieldValue struct {
-	Value string
+	Value string  `json:"value"`
-	Count int64
+	Count int64   `json:"count"`
-	Score float64
+	Score float64 `json:"score"`
 }
 
 // SearchFieldValues handles field-specific search for autocomplete dropdowns