feat: optional idle-leave-after-playback timeout

[seitzbg]

Summary

• Adds an optional, default-off IDLE_LEAVE_TIMEOUT (seconds). When playback ends — a video finishing on its own or !stop — and nothing new starts within the timeout, the bot leaves voice and posts a short note. Unset / empty / 0 keeps current behavior (the bot stays in the channel).
• Per-guild asyncio timer armed from the playback-cleanup (finally) blocks in cogs/stream.py (covers both the camera path and the go-live path). A fire-time guard re-checks the bot is genuinely idle (stream_tasks/live_connections empty, vc not playing, still connected) before disconnecting, so it never cuts off active or resumed playback. The go-live guard relies on stream_tasks/live_connections because go-live audio doesn't go through vc.play().
• Starting new playback re-arms the timer (replacing any prior one); !leave and the auto-leave-on-empty handler cancel it. Pending timers are also cancelled on bot shutdown (SlopSoil.close()).

Changes

• cogs/stream.py: _idle_leave_timeout, _format_idle, schedule_idle_leave, cancel_idle_leave, _idle_leave_after, _idle_leave_now; arm in both finally blocks.
• bot.py: new idle_leave_tasks: dict[int, asyncio.Task], cancelled in close().
• cogs/voice.py: cancel the timer on !leave and auto-leave-on-empty.
• .env.example + README.md: document IDLE_LEAVE_TIMEOUT.
• tests/test_idle_leave.py: new unit tests.

Test Plan

• pytest — full suite green, including new tests/test_idle_leave.py (config parsing, duration formatting, schedule/re-arm/cancel, idle-guard no-ops, and !leave/auto-leave cancellation wiring).
• Manual: IDLE_LEAVE_TIMEOUT=30, play a short clip, let it finish → bot leaves after ~30s and posts "left after 30 sec idle".
• Manual: !stop mid-clip → bot leaves after the timeout; start new playback before it → bot stays.
• Manual: !leave before the timeout → no spurious idle message; unset/0 → never auto-leaves (current behavior).

Merge note

Standalone on main — not stacked, no dependency on #14 or #15. It does touch the README config table / .env block and cogs/stream.py, which #14 (stream profile / packet pacing) also touches, so whichever of #14 / #16 merges second will need a trivial rebase.

[dev-topsoil]

@seitzbg this PR has been sitting stale as a draft for a while and the repo has gone through quite a few changes to the project structure since which may make a merge quite annoying.

I'm going to close the PR for now just for housekeeping reasons. If you want to get your clanker to re-do the PR I'm more than willing to add this feature to Slopsoil.

Also for what it's worth, I did try and reach out via email about linking up on Discord to discuss further features for the project but didn't hear anything back. I was out of town for a while so maybe I just didn't notice the reply or it got caught in a filter. Didn't want you to think I was ignoring you. The author info in my commits should be up to date if you want to reach out that way.