🐛 fix(handlers): 统一错误响应为安全 JSON 并补参数校验

- 新增 respondError：JSON 安全编码，客户端错误返回 400，服务端错误返回 500
- 收编 book/content/media/category 中 13 处 fmt.Fprintf 内联 JSON，修复错误信息含引号/换行时产出非法 JSON 的问题
- search/detail/catalog/multi-detail/content/multi-content/item-summary/audio/video 补必需参数校验，缺参返回 400
- search 的 offset/tab_type 非数字时返回 400，不再静默置 0
- 补齐 multi-detail/multi-content 的请求统计计数
- 新增 error_response_test.go：3 个测试锁定错误响应行为
- openapi.yaml 新增 ServerError 响应组件，15 个业务接口补 400/500 响应定义

BREAKING CHANGE: 业务接口错误不再返回 HTTP 200，缺参返回 400，上游失败返回 500

internal/handlers/book.go

@@ -4,7 +4,6 @@ import (
 	"fanqie/internal/fqapi"
 	"fmt"
 	"net/http"
-	"strconv"
 )
 
 // HandleSearch 搜索处理
@@ -16,22 +15,29 @@ func HandleSearch(w http.ResponseWriter, r *http.Request) {
 	if query == "" {
 		query = r.URL.Query().Get("key")
 	}
-
-	offsetStr := r.URL.Query().Get("offset")
-	offset := 0
-	if offsetStr != "" {
-		offset, _ = strconv.Atoi(offsetStr)
+	if query == "" {
+		respondError(w, "缺少 query 参数", true)
+		return
 	}
 
-	tabTypeStr := r.URL.Query().Get("tab_type")
-	tabType := 3
-	if tabTypeStr != "" {
-		tabType, _ = strconv.Atoi(tabTypeStr)
+	offset, err := parseIntParam(r.URL.Query().Get("offset"), "offset")
+	if err != nil {
+		respondError(w, err.Error(), true)
+		return
+	}
+
+	tabType, err := parseIntParam(r.URL.Query().Get("tab_type"), "tab_type")
+	if err != nil {
+		respondError(w, err.Error(), true)
+		return
+	}
+	if tabType == 0 {
+		tabType = 3
 	}
 
 	result, err := fqapi.HandleBookSearch(query, offset, tabType)
 	if err != nil {
-		fmt.Fprintf(w, `{"code": -1, "msg": "搜索失败: %s"}`, err.Error())
+		respondError(w, "搜索失败: "+err.Error(), false)
 		return
 	}
 
@@ -43,11 +49,15 @@ func HandleDetail(w http.ResponseWriter, r *http.Request) {
 	incrementStat("total", "detail")
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 
-	bookID := r.URL.Query().Get("book_id")
-
-	result, err := fqapi.HandleGetDetail(bookID)
+	params, err := validateParams(r, "book_id")
 	if err != nil {
-		fmt.Fprintf(w, `{"code": -1, "msg": "获取详情失败: %s"}`, err.Error())
+		respondError(w, err.Error(), true)
+		return
+	}
+
+	result, err := fqapi.HandleGetDetail(params["book_id"])
+	if err != nil {
+		respondError(w, "获取详情失败: "+err.Error(), false)
 		return
 	}
 
@@ -59,11 +69,15 @@ func HandleCatalog(w http.ResponseWriter, r *http.Request) {
 	incrementStat("total", "catalog")
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 
-	bookID := r.URL.Query().Get("book_id")
-
-	result, err := fqapi.HandleGetCatalog(bookID)
+	params, err := validateParams(r, "book_id")
 	if err != nil {
-		fmt.Fprintf(w, `{"code": -1, "msg": "获取目录失败: %s"}`, err.Error())
+		respondError(w, err.Error(), true)
+		return
+	}
+
+	result, err := fqapi.HandleGetCatalog(params["book_id"])
+	if err != nil {
+		respondError(w, "获取目录失败: "+err.Error(), false)
 		return
 	}
 
@@ -72,13 +86,18 @@ func HandleCatalog(w http.ResponseWriter, r *http.Request) {
 
 // HandleMultiDetail 批量详情处理
 func HandleMultiDetail(w http.ResponseWriter, r *http.Request) {
+	incrementStat("total", "detail")
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 
-	bookID := r.URL.Query().Get("book_id")
-
-	result, err := fqapi.HandleGetMultiDetail(bookID)
+	params, err := validateParams(r, "book_id")
 	if err != nil {
-		fmt.Fprintf(w, `{"code": -1, "msg": "批量获取详情失败: %s"}`, err.Error())
+		respondError(w, err.Error(), true)
+		return
+	}
+
+	result, err := fqapi.HandleGetMultiDetail(params["book_id"])
+	if err != nil {
+		respondError(w, "批量获取详情失败: "+err.Error(), false)
 		return
 	}