fix: 修复资源不存在/已解散返回错误码及网关502问题

修复好友申请不存在时返回正确的 ErrNotFound 避免解引用 panic，新增通用函数判断群不存在或已解散并返回对应错误码，更新文档统一资源不存在类错误语义，避免服务端返回5xx错误。

server/im_http.go

@@ -4427,6 +4427,10 @@ func serveIMGroupDetail(wrt http.ResponseWriter, req *http.Request, topic string
 		return
 	}
 	if selfSub == nil || !(selfSub.ModeGiven & selfSub.ModeWant).IsJoiner() || !(selfSub.ModeGiven & selfSub.ModeWant).IsReader() {
+		// 群不存在（404）或已解散（410）要与"存在但无权限"（403）区分开。
+		if selfSub == nil && writeIMGroupMissingIfGone(wrt, req, principal.TenantID, topic) {
+			return
+		}
 		writeIMError(wrt, req, http.StatusForbidden, 30005, imReasonPermissionDenied, "topic permission required", nil)
 		return
 	}
@@ -9244,12 +9248,33 @@ func requireIMGroupMember(wrt http.ResponseWriter, req *http.Request, topic stri
 		access = sub.ModeGiven & sub.ModeWant
 	}
 	if sub == nil || !access.IsJoiner() || (needRead && !access.IsReader()) {
+		// 群不存在（404）或已解散（410）要与"存在但无权限"（403）区分开。
+		if sub == nil && writeIMGroupMissingIfGone(wrt, req, principal.TenantID, topic) {
+			return nil, nil, false
+		}
 		writeIMError(wrt, req, http.StatusForbidden, 30005, imReasonPermissionDenied, "topic permission required", nil)
 		return nil, nil, false
 	}
 	return principal, sub, true
 }
 
+// writeIMGroupMissingIfGone 在群不存在或已删除时写出 404/410 并返回 true；查询失败时返回 false 保持原错误路径。
+func writeIMGroupMissingIfGone(wrt http.ResponseWriter, req *http.Request, tenantID types.TenantID, topic string) bool {
+	group, err := store.Topics.Get(tenantID, topic)
+	if err != nil {
+		return false
+	}
+	if group == nil {
+		writeIMError(wrt, req, http.StatusNotFound, 30004, imReasonResourceNotFound, "group not found", nil)
+		return true
+	}
+	if group.State == types.StateDeleted {
+		writeIMError(wrt, req, http.StatusGone, 30004, imReasonGroupDissolved, "group dissolved", map[string]any{"topic": topic})
+		return true
+	}
+	return false
+}
+
 func authenticateTopicReader(wrt http.ResponseWriter, req *http.Request, topic string) (*imPrincipal, string, bool) {
 	principal, ok := authenticateIMHTTPBearer(wrt, req)
 	if !ok {