fix: prevent SSE listener leak in console-logs stream

[zerray]

Summary

MaxListenersExceededWarning: Possible EventTarget memory leak detected. 51 line listeners added to [EventEmitter] (same for clear) appears after many client reconnects on the console-logs SSE endpoint. Node memory grows slowly alongside the listener count.

Root cause

src/app/api/translator/console-logs/stream/route.js attaches line / clear listeners on the singleton consoleEmitter inside ReadableStream.start(), and only removes them in cancel(). In Next.js 16 / Node fetch runtime, ReadableStream.cancel() is not reliably invoked when the client aborts the connection (tab close, navigation, reload), so listeners accumulate every time the dashboard reconnects. The buffered-logs emitter has setMaxListeners(50), so the warning fires on the 51st leaked connection.

request.signal (the AbortSignal on the Request) does fire on client disconnect — it is the authoritative disconnect source here.

Fix

• Add request.signal.addEventListener("abort", cleanup, { once: true }) as the primary disconnect hook.
• Keep cancel() as a fallback.
• Make the cleanup idempotent via a state.closed guard so abort, cancel, and enqueue-failure paths can all call it safely.
• Centralize emitter.off(...) + clearInterval(keepalive) inside the single cleanup() function.

No behavior change for clients; only server-side listener bookkeeping is affected. The usage/stream SSE route has the same pattern and is a candidate for the same fix in a follow-up — this PR keeps the change minimal and focused on the one route where the warning is observed.

Test plan

• Start the app, open the dashboard, then reload the page 60+ times (or open/close the console-logs drawer rapidly)
• listenerCount("line") on the emitter should return to ~1 after each client disconnect, not accumulate
• No MaxListenersExceededWarning in server logs
• Log lines still stream live to the UI; clear action still reaches connected clients
• 25s keepalive ping still fires on idle connections; interval is cleared on disconnect

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