Gateway Chat Stream

Streaming model gateway. Emits crowe-stream v0 events as SSE.

Behind CROWE_STREAM_ENABLED so the rollout is reversible without a redeploy. Plan gating mirrors /chat exactly so streamed access can never exceed the user's tier.

Token accounting note: the v0 renderer counts SDK content deltas (one increment per feed call), which is approximately the completion_tokens count but does not include prompt_tokens. We record what we have on the done event; v1 of the protocol adds the real provider usage block (gap #3 in the spec) at which point this becomes accurate. The endpoint is dogfood-only until that lands.