fix(node): stop losing transcode slots, and reap ffmpeg without deadlocking

Reported from a phone: play a video, close the viewer, open another — the second
hangs and the third is refused. Three separate causes, found by instrumenting
rather than guessing, after two fixes that addressed real but different bugs.

A task nobody holds can be collected mid-flight. asyncio keeps only a weak
reference, so `ensure_future` with the result discarded may be garbage-collected
while running — "Task was destroyed but it is pending!" — and `_stream_video`
never reached the exit of its `async with sem`. `_spawn` holds every background
task; all nineteen call sites go through it.

Losing the peer must stop its work. The connectionstatechange handler popped the
session from a dict and nothing else, so a closed tab went on transcoding for
the full 120 s credit timeout. Measured in the log: 91 s of ffmpeg after the
connection closed. `shutdown_tasks()` now runs on the way out, and the credit
wait checks the channel before sleeping and polls in slices instead of once.

And `await proc.wait()` after `kill()` still deadlocks. ffmpeg outruns a
credit-paced viewer and fills the stdout pipe; stop reading it and the transport
cannot finish closing, SIGKILL or not. Measured against the live node with a
169 MB video, closing the viewer after 20 segments and asking for the next one:
15.1 s then "Server busy" before, 0.1 s / 0.0 s / 0.0 s after.

Chunk replies wait for room on the channel. Eight megabyte-sized chunks answered
as they arrived queued 8 MB with nothing watching — measured at 7.3 MB of
bufferedAmount in milliseconds. Fine on a LAN, minutes of head-of-line delay on
a busy link.

Upload names accept any script. The rule was ASCII-only, so `été.txt` was
refused — and so was `rapport (1).pdf`, which is the form `_free_name` produces
itself, meaning the node rejected names it had chosen. Widened to Unicode with
the C5a and H2 protections intact, plus a refusal of names that lie about
themselves: trailing space or dot, and the right-to-left override. Errors now
name the file, so one bad name no longer fails every upload in flight.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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