feat(server): 免费版账户级分钟卡控 + 累加式看广告加时(#21 后端)

免费版此前形同虚设:ConnectNode 每次连接发固定 daily_minutes×1min TTL、
从不扣减已用,重连即崭新 10 分钟,日额度从未强制。改为账户级(全设备共享)
真卡控 + 累加式看广告加时:

- migration 000020: usage_daily 加 ad_bonus_minutes(sqlite+mysql)。
  当日额度 = plans.daily_minutes + ad_bonus_minutes;剩余 = 额度 − minutes_used。
- usage.store.AddAdBonusMinutes: 事务内累加封顶(替代布尔 MarkAdUnlocked),
  返回新总额+本次实际加时;GetDay/GetUsageRange 补读 ad_bonus_minutes。
- usage.service.UnlockAd: verify+nonce 后 +adBonusPerAd(10) 封顶 adDailyBonusCeiling(120),
  返回 granted+remaining;TodaySummary 加 MinutesCap/AdBonusMinutes。
- usage.ads DevVerifier: 放行式占位校验(nonce 防重放仍生效),接真 AdMob 前
  让看广告加时流程可端到端跑通;main.go 按 ADS_DEV_MODE/默认装配。
- ads/unlock: 204 → 200 返回 {granted_minutes, minutes_remaining}。
- ConnectNode 免费门: 读 AccountDayMinutes(used,bonus) → remaining≤0 拒 QUOTA_EXHAUSTED,
  否则 TTL=remaining(凭证到点硬切断兜底)。nodes.store 加 AccountDayMinutes。
- /me: 补 ad_bonus_minutes,quota_today_min 分母改 daily+bonus,加 quota_cap_min。
- quota.CheckFreeConnect 同步累加语义(免费基础 10 分钟不再需先看广告)。
- openapi + 契约/迁移版本/集成测试同步;新增 SQLite AddAdBonusMinutes 单测。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/usage/handler.go

@@ -144,9 +144,16 @@ type adsUnlockRequest struct {
 	AdToken  string `json:"ad_token"`
 }
 
-// ServeHTTP implements http.Handler.  On success it returns 204 No Content
-// (matching the OpenAPI contract), including the idempotent already-unlocked
-// case.
+// adsUnlockResponse reports the minutes granted by this rewarded-ad view and
+// the account's new remaining minutes for today, so the client can refresh its
+// quota without a second /me round-trip.
+type adsUnlockResponse struct {
+	GrantedMinutes   int `json:"granted_minutes"`
+	MinutesRemaining int `json:"minutes_remaining"`
+}
+
+// ServeHTTP implements http.Handler.  On success it returns 200 with the granted
+// minutes and new remaining quota (免费版累加式看广告加时).
 func (h *AdsUnlockHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodPost {
 		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
@@ -168,13 +175,15 @@ func (h *AdsUnlockHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	_, apiErr := h.svc.UnlockAd(r.Context(), userID, req.DeviceID, req.AdToken)
+	granted, remaining, apiErr := h.svc.UnlockAd(r.Context(), userID, req.DeviceID, req.AdToken)
 	if apiErr != nil {
 		apierr.WriteJSON(w, adsErrorStatus(apiErr.Code), apiErr)
 		return
 	}
 
-	w.WriteHeader(http.StatusNoContent)
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(http.StatusOK)
+	_ = json.NewEncoder(w).Encode(adsUnlockResponse{GrantedMinutes: granted, MinutesRemaining: remaining})
 }
 
 // adsErrorStatus maps an ads-unlock error code to an HTTP status.