feat: record which tier a call actually ran at

Twenty-five columns and not one of them answered "which tier was this?",
so the question the whole issue exists to settle - does a higher tier buy
anything - had no way to group its data.

The three emit entry points deliberately disagree, the way sampling
already does. A successful attempt records what the transport actually
sent: with EFFORT_FALLBACK=nearest a request for medium goes out as low,
and recomputing here would file the row under a tier that never left the
process. A failed attempt has no response to read, so it falls back to
the requested tier - which is exactly right for the tier errors that are
rejected before any HTTP happens, because the rejected tier is the
signal. Cache hits and terminal failures have no chosen source at all,
so a source-level tier is not a thing they could report.

emit_attempt now demands to be told whether the path reasons at all.
Embedding and OCR share the emitter but never send reasoning parameters;
without the flag a source that mistakenly carries ENABLE_THINKING would
hang a tier on a call that could not possibly have run at one.

The value lands as a plain str. StrEnum is a str subclass and asyncpg
promises nothing about encoding subclasses, and a telemetry write that
fails is only a warning - Postgres would just quietly lose the column.
NULL means nobody declared a tier, which is not the same statement as
'none', and the two must never be folded together.

README.md

@@ -19,7 +19,7 @@
 | 响应缓存 | Redis/内存;key 含 model + messages 摘要 + namespace(缓存隔离单位)+ salt + 采样参数,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 |
 | 推理可观测性 | "这次到底推理没推理"由多信号裁定(推理正文压倒 usage 明细),三态落在 `LLMResponse.thinking_observation`:`observed` / `absent` / `unknown`——**`unknown` 是"本次判不出",不是"没推理"**;请求方向与实测观测矛盾时按 `(模型, 方向)` 各告警一次(能力表过期、开启未生效、注入了却观测不到);裁定结果随遥测落库 |
-| 遥测与成本 | 每次调用(含缓存命中与失败)必录 25 字段;SQLite / Postgres 后端(表已存在时**不需要** schema 建表权限,最小权限账号可直接用);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 |
+| 遥测与成本 | 每次调用(含缓存命中与失败)必录 26 字段;SQLite / Postgres 后端(表已存在时**不需要** schema 建表权限,最小权限账号可直接用);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 |
 | 遥测的资源与降级 | Postgres 池**闲时占 0 条连接**、忙时上限可配(`PGW_TELEMETRY_PG_POOL_MAX`,缺省 4),每次写入有硬预算(`PGW_TELEMETRY_PG_WRITE_TIMEOUT_S`,缺省 5s);后端不可用是**可恢复的降级**(冷却 60s 后自动重试,DBA 建完表/放开权限即自愈),永久失能只留给 DSN 本身写错;降级状态可编程查询——`client.telemetry_status` 给出 `degraded`/`fatal`/`reason`/`dropped_rows` 等只读快照,不必再靠人工对账。**对账要同时看 `degraded` 与 `dropped_rows`**: 池饱和超预算丢的行走行级丢弃,`degraded` 保持 `False`(后端没挂,是本进程并发超了),只按 `degraded` 告警会看不见这一类丢行——而它恰是 `pool_max` 配小了的唯一信号 |
 | 调用方维度 | 每次调用可带 `tenant_id`(遥测表的真实列,可挂 RLS、可建复合索引)与 `meta`(≤16 个自定义 KV);四个公共方法全覆盖,校验超限即报错;**库只交付列,不启用 RLS、不建索引** |
 | 遥测表治理 | `llm_calls` 是**下游的表**:PG 侧缺省**不再自动 `ALTER` 补列**(`PGW_TELEMETRY_SCHEMA_MODE` 三态,不设则 sqlite→auto、postgres→manual),manual 档点名缺列并按现有列裁剪写入;`telemetry_schema_sql(backend)` 自取可粘进迁移文件的建表/补列 SQL;`PGW_TELEMETRY_TEXT_CAP` 限正文长度(**不设 = 存全文**);保留期与访问控制走[生产部署 DDL 模板](#生产部署-ddl-模板postgresql)加 `tools/telemetry_retention.py` |