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/quota.go

@@ -11,16 +11,16 @@ import (
 const defaultFreeDailyMinutes = 10
 
 // CheckFreeConnect enforces the free-plan connect gate and returns the user's
-// remaining minutes for today (UTC).  It is called by #5's connect endpoint to
-// derive the free credential's TTL.
+// remaining minutes for today (UTC).  The quota is account-wide (shared across
+// all devices).
 //
-// Rules:
+// Rules (免费版累加式看广告加时):
 //   - plan.ad_gate == false (pro/team): not minute-gated.  Returns Unlimited
 //     when daily_minutes is NULL, otherwise the remaining minutes for the day.
-//   - plan.ad_gate == true (free): today's ad_unlocked_at must be set and
-//     minutes_used must be below daily_minutes (free = 10).  Returns the
-//     remaining minutes; otherwise a bilingual semantic error
-//     (AD_NOT_UNLOCKED / QUOTA_EXHAUSTED).
+//   - plan.ad_gate == true (free): the day's allowance = daily_minutes (free=10)
+//     + ad_bonus_minutes (accumulated by watching rewarded ads).  No ad is
+//     required to connect at all — the base 10 minutes are always available.
+//     Returns allowance − minutes_used; QUOTA_EXHAUSTED when that reaches 0.
 func (svc *Service) CheckFreeConnect(ctx context.Context, userID int64) (remainingMinutes int, apiErr *apierr.Error) {
 	plan, err := svc.store.EffectivePlan(ctx, userID)
 	if err != nil {
@@ -51,16 +51,19 @@ func (svc *Service) CheckFreeConnect(ctx context.Context, userID int64) (remaini
 		return remaining, nil
 	}
 
-	// Free plan: require ad unlock + remaining minutes.
+	// Free plan: allowance = base daily + ad bonus; remaining = allowance − used.
 	day, err := svc.store.GetDay(ctx, userID, utcToday())
 	if err != nil {
 		return 0, apierr.ErrInternal
 	}
-	if day == nil || !day.AdUnlockedAt.Valid {
-		return 0, apierr.ErrAdNotUnlocked
+	allowance := limit
+	used := 0
+	if day != nil {
+		allowance = limit + day.AdBonusMinutes
+		used = day.MinutesUsed
 	}
-	if day.MinutesUsed >= limit {
+	if used >= allowance {
 		return 0, apierr.ErrQuotaExhausted
 	}
-	return limit - day.MinutesUsed, nil
+	return allowance - used, nil
 }