feat(v0.6.1): heartbeat-stream /query/ + /upload/ (survive mobile/Tailscale tunnel drops)

[Hashevolution]

Summary

Operator (phone over Tailscale): a long query returned "Failed to fetch" before the answer, and the composer upload chip stayed stuck even though the server finished (entities formed). Shared root cause: a request that holds an idle connection for 30–90s (full RAG query / image ingest with vision OCR) is evicted by the mobile network or a Tailscale re-handshake. The blocking call inside the async handler also pinned the event loop, so /trace/poll couldn't stream reasoning stages live.

Fix: core/http_heartbeat.stream_json_with_heartbeat runs the blocking work in a worker thread (frees the loop → /trace/poll streams stages live) and emits JSON-insignificant whitespace every ~12s while it runs, so bytes keep flowing and the tunnel stays up. The heartbeat is leading whitespace, which r.json() / JSON.parse ignore → no client change. current_trace_id (ContextVar) propagates into the thread via anyio.to_thread, preserving trace correlation.

Bonus

Threading the blocking pipeline also fixes the "reasoning not reflecting live" symptom — the event loop is now free to serve /trace/poll during the request.

Verification

• tests/test_http_heartbeat.py 4/4: stream+parse, error-in-body, fast-path, contextvar propagation.
• routes.query / routes.auth import OK; slow blocks wrapped, validation (400/413/403) still raises proper status before streaming.
• test_measurement_critical_surfaces 33/33.
• Live end-to-end pending on the operator's server (real query/upload over the tunnel) — the helper + propagation are unit-tested; the route wraps are structural.

Trade-off

A rare slow-path error (after streaming starts, status already 200) now surfaces in the body ({"answer":"","error":...}) instead of a 5xx; the file is still saved + marked failed server-side. Validation errors are unaffected (raised before streaming).

Quality delta

Quality delta: exempt (label: feat) — transport-robustness wrapper on two routes + a new core/http_heartbeat util; core/retrieval + core/graph traversal + core/reasoning 0 lines.

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