Idle sessions should not pay the same poll/liveness cost as the active turn

[15273]

Symptom

Many sessions sit in the sidebar. The panel still does a full GET /sessions (and per-session get_session_state on the bridge) on every poll. When the active chat is working that poll is every 2.5s (SESSION_POLL_ACTIVE_MS in useSessionManager.ts). Idle chats should not keep the same request pattern.

This also shows up as bridge jank: the comment in useSessionManager.ts already says mass /transcript fetches starve WebSockets. Transcript sync was restricted to the active session; the list poll + liveness path was not fully tightened.

What already exists

• Fast vs idle poll interval is based on the active session only (SESSION_POLL_ACTIVE_MS vs SESSION_POLL_IDLE_MS).
• /transcript merge is active-session-only.
• WS keepalive: collectWsKeepOpen in extension/src/shared/wsKeepalive.ts — active + turnRunning + persistent automation sessions.

What to change

Goal: only sessions that need it should trigger expensive work.

Concrete directions (pick one, measure, then the next):

1. Cheap GET /sessions — listing metadata for idle chats should not call get_session_state / liveness for every name. Example: ?live=1 or skip liveness unless active / turn_running / recently used. See list_sessions in devscope_bridge/session_http.py (loops all stored sessions today).
2. Do not open WS for idle user chats (already partly true; audit automation + background workers).
3. Do not recoverPending / heal WS in a fan-out across every name on every poll.
4. Document the invariant: active chat + in-flight turns (+ intentional automation) may poll/stream; everyone else is list-only until selected.

Done when

• Idle sessions do not cause per-session process/liveness work on the 2.5s path (pytest on the list endpoint or get_session_state call count).
• Switching to an idle session still shows correct turn_running / awaiting cards within one poll.
• Active turn streaming is unchanged (no extra dropped WS frames).

Good first slice: add a query flag or skip _liveness_fields for sessions that are not active and not is_turn_running, with a test that a large idle list stays cheap.