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BFF scalability: cluster mode, backpressure, streaming, horizontal-readiness #1616

Description

@Xore

Split out of #1610 (per Xore) so it's tracked separately from the worker migration, which is now complete (#1612/#1613/#1614/#1615).

BFF scalability (hard requirement)

The BFF must scale properly and never get overwhelmed — it carries no compose resource limit:

  • Node cluster mode / worker_threads for CPU-bound composition; the event loop stays free
  • Backpressure end-to-end: bounded queues for fan-out to the Rust tier, 429/shed-load before collapse, per-route concurrency caps
  • Streaming responses (no buffering whole payloads), keep-alive pooling to the Rust service
  • Horizontal-ready: no in-process state that prevents running N BFF instances later (session in redis, prefetch cache shareable)
  • Load-test gate: sustained SSE fan-out + burst navigation without p99 regression

None of this has been implemented yet — #1610's worker-migration portion is done, but this section was never started.

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