feat: wire the telemetry text cap through settings

`PGW_TELEMETRY_TEXT_CAP` now reaches the emitter on every assembly path.
Unset means no truncation, which stays the default: a truncated row is
no longer audit evidence and cannot be replayed, and downstreams rely on
that today. The flip side — contracts and bids sitting in `llm_calls`
indefinitely, multi-tenant — is spelled out in `.env.example` so readers
can weigh both.

All three `from_settings` paths are wired (chat, embedding, OCR): they
write the same table, so capping only chat would leave half of it
uncontrolled. `TelemetryEmitter.__init__` now rejects `text_cap <= 0`;
it is the single point where the three clients converge, so the direct
construction path — a public assembly route the settings guard never
sees — is covered too. `0` would otherwise reduce every body to a bare
elision marker.

.env.example

@@ -66,6 +66,13 @@ PGW_TELEMETRY_BACKEND=none               # sqlite | postgres | none(必填)
 #                                               # sqlite 则是下游自己的本地文件(runs/*.db):没有 DBA、没有迁移工具、
 #                                               # 没有第二个系统碰它,ALTER 是毫秒级元数据操作,强加手工 SQL 步骤是净损失。
 # PGW_TELEMETRY_PG_DSN=postgresql://user:pass@host:5432/polygateway  # postgres 时必填;严禁指向在用业务库(实验室约定: 专用库 polygateway)
+# PGW_TELEMETRY_TEXT_CAP=2000                   # 遥测落库正文的字符上限,须 > 0;**不设 = 不截断**(缺省,逐字节留全文)。
+#                                               # 作用于 messages 的每条文本 content、多模态 text part、response 与 thinking;
+#                                               # 超出部分头部保留、尾部换成 `…(略 N 字)`。多模态 image_url 的 sha256 摘要不受影响。
+#                                               # 缺省为何是"不截断": 遥测被下游当**审计证据**用——出了问题要回答"当时到底发了什么",
+#                                               # 也要能拿原样的请求复现与重放;截断后这两件事都做不成,而既有下游正依赖这一行为。
+#                                               # 反面同样要看清: 不截断意味着客户合同、标书全文无限期留在 llm_calls 里,
+#                                               # 多租户下还混在同一张表。真在意留存面的部署应显式设一个上限,并配保留期与访问控制。
 # PGW_PRICING_PATH=config/prices.json           # 可选: {"<model>": {"input_per_1m": x, "output_per_1m": y}};缺省 cost 恒 None
 #                                               # 可选第三档 "cached_input_per_1m": z —— 供应商 prompt cache 命中部分的单价;
 #                                               # 不填即命中部分也按 input 全额计(库不猜折扣率),cost 会偏高