fix(insights): cancel the Coach upstream provider call on client disconnect

The Coach SSE turn ran the provider call to completion before the stream
existed and never wired it to the request's abort signal, so a mid-generation
client disconnect still paid the full upstream token cost. Thread an optional
`signal` through CompletionParams and singleUserTurn into each client's
safeFetch (composed with the per-call timeout via AbortSignal.any), and pass
`request.signal` from the chat route's no-tools path. An early disconnect now
tears the upstream request down instead of generating into a closed
connection. Omitting the signal leaves behaviour unchanged (timeout-only).

src/app/api/insights/chat/route.ts

@@ -567,6 +567,10 @@ Reply now as the assistant, in ${locale === "de" ? "German" : "English"}. Fetch
           user: userPrompt,
           temperature: AI_BUDGETS.coach.temperature,
           maxTokens: AI_BUDGETS.coach.maxTokens,
+          // v1.20.1 — thread the request's abort signal so a mid-generation
+          // client disconnect tears the upstream provider call down instead of
+          // paying the full token cost into a closed connection.
+          signal: request.signal,
         }),
       });
       result = fallback.result;