New text/event-stream endpoints — /o/{offer_id}/events (buyer/seller only)
and /a/{auction_id}/events (public). Each polls the row ~every 1.5s, emits
a `data: {json}` frame on connect and whenever the state-machine state
changes, sends a heartbeat comment, then closes after ~25s so the browser
EventSource reconnects — "bounded" because uWSGI is sync (~16 worker
threads) and a long-lived SSE would starve the pool. Shared helper
lib/sse.py (sse_response / event_stream); it uses its own short-lived DB
session per poll (request.dbsession is already closed by pyramid_tm by the
time the streaming generator runs). Timings come from settings
(app.sse.hold_seconds / app.sse.poll_interval_seconds; test.ini sets them
tiny so the streaming tests finish in ~0.06s).
Client: auction.js opens the EventSource and feeds each frame into its
existing applyState(); it falls back to polling /a/{id}.json every 5s
where EventSource is unavailable. offer.js opens the EventSource on the
offer page and reload()s on a state change (the whole layout depends on
state / can_act). offer.j2 carries data-offer-state. Caddy auto-detects
text/event-stream and stops buffering — no Salt change.
Tests: 4 new functional tests (both endpoints stream the right
content-type + a state frame; 404 for outsiders / unknown ids). 994 passed.