fix(tts): 服务端 tts_cache 去重,防传 null 强制重合成刷 TTS 账单

- 合成前先查 tts_cache 集合,命中则直接 getTempFileURL,不重复合成
- 合成后写 tts_cache,相同参数组合只合成一次
- 集合不存在时 catch 走原合成路径不崩(需在云开发控制台建 tts_cache 集合启用去重)

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

cloudfunctions/tts/index.js

@@ -80,12 +80,23 @@ exports.main = async (event) => {
 
   // If the client passed a cached fileID, try to get a fresh temp URL
   // from it — this is the fast path (no synthesis, no upload).
-  if (fileID) {
+  const db = cloud.database()
+  const cacheKey = `${promptKey}-${voiceType || 'default'}-v${volume != null ? volume : 0}-s${speed != null ? speed : 0}`
+
+  // 解析 fileID:优先 client 传入,否则查服务端 tts_cache,防 client 传 null 强制重合成(刷 TTS 账单)
+  let resolvedFileID = fileID || null
+  if (!resolvedFileID) {
     try {
-      const urlRes = await cloud.getTempFileURL({ fileList: [fileID] })
+      const r = await db.collection('tts_cache').doc(cacheKey).get()
+      if (r.data && r.data.length && r.data[0].fileID) resolvedFileID = r.data[0].fileID
+    } catch (e) {}  // 集合/文档不存在,走合成
+  }
+  if (resolvedFileID) {
+    try {
+      const urlRes = await cloud.getTempFileURL({ fileList: [resolvedFileID] })
       const url = urlRes.fileList[0].tempFileURL
       if (url) {
-        return { success: true, audioUrl: url, fileID, text, cached: true }
+        return { success: true, audioUrl: url, fileID: resolvedFileID, text, cached: true }
       }
     } catch (e) {
       // fileID stale or file deleted — fall through to re-synthesize
@@ -101,6 +112,8 @@ exports.main = async (event) => {
     if (!url) {
       return { success: false, error: 'getTempFileURL returned empty URL', text }
     }
+    // 写服务端 tts_cache,下次相同参数直接命中,不重复合成
+    try { await db.collection('tts_cache').doc(cacheKey).set({ data: { fileID: newFileID, updatedAt: db.serverDate() } }) } catch (e) {}
     return { success: true, audioUrl: url, fileID: newFileID, text, cached: false }
   } catch (e) {
     return { success: false, error: e.message, text }