Rename idle reaper timeout to minutes

[lewtun]

Summary

• replace REAPER_IDLE_HOURS with REAPER_IDLE_MINUTES defaulting to 15
• update per-user capacity errors to refer to live sessions and idle release timing
• add tests for the default timeout and capacity message wording

Tests

• uv run pytest tests/unit/test_session_reaper.py
• uv run ruff check .
• uv run ruff format --check .

[github-actions]

Claude finished @lewtun's task in 47s —— View job

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Review: PR #305 — Rename idle reaper timeout to minutes

No blocking issues — 2 P2

Clean rename. REAPER_IDLE_HOURS → REAPER_IDLE_MINUTES is applied consistently, no stray references remain anywhere in the repo (grep confirms only the new symbol in backend/session_manager.py and tests/), timedelta is already imported in the test, and the new tests correctly cover both the default and the reworded capacity message.

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P2 — Stated wait time can understate actual release by up to one reaper interval · backend/session_manager.py:845-849

The message tells the user to "wait {REAPER_IDLE_MINUTES:g} minutes after your last activity for an idle session to be released." But eviction is driven by the periodic reaper loop (REAPER_INTERVAL_S, default 300s — session_manager.py:148), which only sweeps sessions whose idle age has already crossed REAPER_IDLE (cutoff = datetime.utcnow() - REAPER_IDLE, session_manager.py:1083). So worst-case wall-clock is REAPER_IDLE_MINUTES + REAPER_INTERVAL_S ≈ 20 min, not 15. Not worth blocking — the message is directionally correct and "after your last activity" softens it — but if you want it precise, phrase as "within a few minutes after ~15 minutes of inactivity" or similar.

P2 — Default is now 8× more aggressive (2h → 15m) · backend/session_manager.py:147

Flagging the behavior change for visibility, not as a defect: idle live sessions now leave the live pool after 15 min instead of 2 h. Functionally safe since they stay fully resumable from Mongo (per the comment at session_manager.py:143-146), and it does free per-user/global slots faster, which seems to be the intent. Just confirm prod has REAPER_IDLE_MINUTES set explicitly if 15 min isn't the desired live-pool TTL there.

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Tests look good — the per-user-cap test asserts each clause of the new message independently, which will catch future wording drift. LGTM to merge.