feat(billing): reasoning 价接入 DSL 与规则编辑器

上一提交把链路打通后，reasoning_price 只能靠手改 prices 生效 —— 而 prices 是
编译产物，不是人能编辑的东西。这条补上让它真正可用：

- BillingToken 加 reasoning 字段（USD/百万，与 prompt/completion 同单位）
- compile 只在规则显式声明时写 reasoning_price，缺省时字段不出现
- 规则编辑器的模型行加「思考 $/M」输入框，含新增行的模板

缺省时「不出现」是刻意的：写 0 会被 billing 插件读成「思考免费」，
每个来自该模型的请求都会静默少算。判据专门钉住这一点，变异成「缺省写 0」
即变红。

输入框 placeholder 写明「留空=按输出价计（OpenAI/DeepSeek/Anthropic 都这样）」，
因为一个没有解释的空输入框会被当成「必填但我不知道填什么」。

判据还要求 m-r 出现两次 —— 只改已有行的模板而不改「+ 模型」新建的行，
会让新加的模型在下一次保存时静默丢掉这个价。

生产验证：PUT 一个 reasoning: "1.80" 的规则，编译产物里出现
reasoning_price: 1.8000000000000001e-06（USD/token）。测完已把生产配置恢复为
6 条规则（备份 config.yaml.bak-billing-20261004-122151 为基准重建，
20 个源数量核对一致）。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>
(cherry picked from commit b93799a919)

internal/lua/plugins/billing.lua

@@ -1031,7 +1031,9 @@ plugin.ui = {
       monthly: "Monthly fee", peakMult: "Peak multiplier",
       peakHours: "Peak hours (UTC)", peakDays: "Peak days (UTC 1=Mon)",
       models: "Per-model prices", addModel: "+ model",
-      mFree: "free", mToken: "token", mSub: "subscription", mUnpriced: "unpriced",
+
+      reasoning: "think $/M",
+      reasoningHint: "Thinking tokens are billed as output unless you set a different rate here.",      mFree: "free", mToken: "token", mSub: "subscription", mUnpriced: "unpriced",
       noProfile: "No profile yet — create one to start pricing.",
       noRules: "No rules in this profile",
       warnNoSource: "matches no configured source",
@@ -1053,7 +1055,9 @@ plugin.ui = {
       monthly: "月费", peakMult: "峰段倍数",
       peakHours: "峰段小时 (UTC)", peakDays: "峰段星期 (UTC 1=周一)",
       models: "按模型价格", addModel: "+ 模型",
-      mFree: "free 免费", mToken: "token 按量",
+
+      reasoning: "思考 $/M",
+      reasoningHint: "留空=思考 token 按输出价计（OpenAI/DeepSeek/Anthropic 都这样）。只有供应商对思考单独定价时才填。",      mFree: "free 免费", mToken: "token 按量",
       mSub: "subscription 订阅", mUnpriced: "unpriced 只统计",
       noProfile: "还没有方案 —— 新建一个开始定价。",
       noRules: "该方案暂无规则",
@@ -1151,6 +1155,12 @@ plugin.ui = {
         "<input class='m-name' value='" + esc(m) + "' style='flex:1 1 90px;min-width:0'>" +
         "<input class='m-p' value='" + esc(t.prompt == null ? "" : t.prompt) +
           "' placeholder='" + esc(T.prompt) + "' style='flex:0 1 62px;min-width:0'>" +
+          // Left BLANK by default on purpose: empty means "bill thinking at the
+          // completion rate", which is what OpenAI, DeepSeek and Anthropic do.
+          // A 0 here would mean "thinking is free" and would under-bill quietly.
+          "<input class='m-r' value='" + esc(t.reasoning == null ? "" : t.reasoning) +
+            "' placeholder='" + esc(T.reasoning) + "' title='" + esc(T.reasoningHint) +
+            "' style='flex:0 1 62px;min-width:0'>" +
         "<input class='m-c' value='" + esc(t.completion == null ? "" : t.completion) +
           "' placeholder='" + esc(T.completion) + "' style='flex:0 1 62px;min-width:0'>" +
         "<button class='ghost small m-del' title='" + esc(T.delRule) + "' style='flex:0 0 auto'>&times;</button></div>";
@@ -1227,6 +1237,20 @@ plugin.ui = {
           return;
         }
         models[n] = { prompt: pv, completion: cv };
+
+          // Optional thinking-token rate, read from the same row. Blank means
+          // "bill reasoning at the completion rate" — what OpenAI, DeepSeek and
+          // Anthropic all do. A 0 would mean "thinking is free", so an empty
+          // field must stay ABSENT from the saved rule rather than becoming 0.
+          var rel = r.querySelector(".m-r");
+          var rtxt = rel ? String(rel.value || "").trim() : "";
+          if (rtxt !== "") {
+            if (isNaN(parseFloat(rtxt))) {
+              bad = n + " (" + T.reasoning + ")";
+              return;
+            }
+            models[n].reasoning = rtxt;
+          }
       });
       if (bad) throw new Error(T.needPrices + ": " + bad);
       if (Object.keys(models).length) out.models = models;
@@ -1417,6 +1441,8 @@ plugin.ui = {
           "<input class='m-name' placeholder='model' style='flex:1;min-width:80px'>" +
           "<input class='m-p' placeholder='" + esc(T.prompt) + "' style='width:96px'>" +
           "<input class='m-c' placeholder='" + esc(T.completion) + "' style='width:96px'>" +
+            "<input class='m-r' placeholder='" + esc(T.reasoning) + "' title='" + esc(T.reasoningHint) +
+              "' style='width:96px'>" +
           "<button class='ghost small m-del'>&times;</button>";
         box.appendChild(row);
       };