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/client.py

@@ -317,6 +317,7 @@ class GatewayClient:
             pricing=PricingTable.from_file(settings.pricing_path)
             if settings.pricing_path is not None
             else None,
+            text_cap=settings.telemetry_text_cap,
             cache=cache if cache is not None else _build_cache(settings),
             cache_namespace=settings.cache_namespace,
             cache_ttl_s=settings.cache_ttl_s,