feat(scheduling): add opt-in "prefer soonest reset" account selection

Adds a use-it-or-lose-it scheduling strategy: prefer accounts whose
session window resets soonest, so near-reset accounts get drained first
instead of accounts whose reset is still far away.

Both schedulers, opt-in, default behavior unchanged:

- Anthropic (gateway_service.go): new GatewaySchedulingConfig
  .PreferSoonestReset flag. When on, the layered load-aware selection
  inserts a filterBySoonestReset stage (priority -> soonest-reset ->
  load -> LRU). Accounts with no active SessionWindowEnd are treated as
  lowest priority; ties fall through to LRU.

- OpenAI/Codex (openai_account_scheduler.go): new "reset" score weight
  in GatewayOpenAIWSSchedulerScoreWeights. Soonest-reset accounts score
  higher; weight defaults to 0 (no effect).

SessionWindowEnd (upstream 5h/quota ResetsAt) is already carried in the
scheduler snapshot, so no snapshot changes are needed.

Documented in deploy/config.example.yaml. Adds unit tests for the
Anthropic filter and the OpenAI reset factor.

deploy/config.example.yaml

@@ -320,6 +320,9 @@ gateway:
       queue: 0.7
       error_rate: 0.8
       ttft: 0.5
+      # use-it-or-lose-it：倾向「会话窗口最早重置」的账号，剩余重置时间越短得分越高。
+      # 0 表示关闭（默认，不改变原有行为）；调大可让即将重置的账号被优先用尽。
+      reset: 0.0
   # OpenAI 高级调度器补充配置
   openai_scheduler:
     # 是否允许 session_hash sticky 在账号健康度恶化时临时逃逸；false 可一键回退旧行为
@@ -421,6 +424,10 @@ gateway:
     # Fallback max waiting queue size
     # 兜底最大排队长度
     fallback_max_waiting: 100
+    # Prefer the account whose session window resets soonest (use-it-or-lose-it).
+    # 负载感知选择时优先用尽「会话窗口最早重置」的账号；false 保持
+    # 原有「优先级 → 负载率 → LRU」行为（默认）。
+    prefer_soonest_reset: false
     # Enable batch load calculation for scheduling
     # 启用调度批量负载计算
     load_batch_enabled: true