docs: correct the telemetry field count and document backpressure

Three README drifts, none of them about issue #8 alone:

- the telemetry row said 18 fields; record_llm_call takes 22 (verified by
  inspect.signature). ports.py claimed 20 in its own docstring, so both
  records of the same fact were stale.
- LLMResponse.cost is hardcoded to None on the chat path (retry.py:519).
  Cost only ever reaches telemetry. The old doc site named this as a known
  trap, so the capability row now says it outright.
- backpressure had no row at all, which is what issue #8 was about.

README.md

@@ -15,9 +15,10 @@
 | 错误分类重试 | 一切失败落入四分类(见下),由分类决定重试/换源/熔断;429 属 pushback 不消耗重试预算;退避含 jitter 且尊重 Retry-After |
 | 熔断 | 双通道(连续失败 + 失败率窗口,健康证据抑制误熔);半开单探针带租约(持有者死亡自动回收);epoch fencing 拒绝迟到写回;开路时长指数递增 |
 | 自适应并发 | AIMD:429 削减、成功缓升,防止打爆上游 |
+| 背压与判死 | 配额满可选等待或快速失败;等待期按双条件判死(本地非生产性等待与全局无进展**同时**超窗)。stall 窗口只计**非生产性**等待(429 退避/配额轮询/熔断冷却),与 `TIMEOUT_S` 无耦合 |
 | 响应缓存 | Redis/内存;key 含 model + messages 摘要 + namespace/租户 + salt,多模态 content 先摘要再 hash(防毒化);可 per-call 绕过(科研重采样) |
 | 流式看门狗 | TTFT / inter-token / 总超时三层活性;thinking token 刷活性不计结果;截断流(缺 `[DONE]`)判瞬时不入缓存 |
-| 遥测与成本 | 每次调用(含缓存命中与失败)必录 18 字段;SQLite / Postgres 后端;按价格表折算成本;多模态内容摘要落库不存原图 |
+| 遥测与成本 | 每次调用(含缓存命中与失败)必录 22 字段;SQLite / Postgres 后端;按价格表折算成本落库(注意 `LLMResponse.cost` 本身恒为 `None`,成本只进遥测);多模态内容摘要落库不存原图 |
 | 结构化输出 | json_repair 修复 / 原生 schema 双策略 + 校验失败有界带反馈重问 |
 | OCR | MonkeyOCR 双端点(文本转录 + 版面解析),bbox 数值防御下沉,逐源健康预检 `check_health()` |
 | Embedding | 分批、维度校验、与 chat 同一治理栈 |