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.

src/polygateway/ocr.py

@@ -573,6 +573,9 @@ class OcrClient:
             backpressure=gw.backpressure,
             quota_full=gw.quota_full,
             telemetry=telemetry if telemetry is not None else _build_telemetry(gw),
+            # OCR 行与 chat 行写同一张 llm_calls;漏传这一条,同表内就一半受控
+            # 一半不受控(issue #12)
+            text_cap=gw.telemetry_text_cap,
         )
 
     @classmethod