Streaming (SSE) natural-language Q&A over a US coordinate

Server-Sent Events variant of /v1/ask, same request body. Emits delta frames as the answer is synthesized (first tokens ~5-7 s), then one terminal final frame carrying the full /v1/ask body, or an error frame on mid-stream failure. Same 110 s deadline and 429 ask_busy back-pressure as the buffered route; failures before the first byte surface as a real HTTP status (with Retry-After when retryable). Set a client timeout of at least 120 s.