fix(node): size the subtitle budget to the file, and extract each track once

Verifying the feature across a library turned up two faults, one of which
broke it outright.

**The extraction budget was a constant and the cost is not.** Extracting
demuxes the whole container, so the file sets the price: measured at 9.8 s per
GB on a library held on an external disk — 36 s for a 3.9 GB title, 71 s for a
7.3 GB one. Against a flat 60 s that worked on most of a library and failed on
the big films, which to whoever is watching one is indistinguishable from a
broken feature. `Avatar.The.Way.of.Water` timed out every time. The budget is
now three times the measured rate per gigabyte, floored at the old 60 s and
capped so no container can pin a transcode slot indefinitely. The client's
own timeout follows it: the node always answers, so that one is a backstop
against a silent peer rather than a deadline for the work.

**Two clicks ran two extractions.** The cache is consulted on the way in, so a
second request arriving while the first was still running missed it and ran
its own — seen in the log as two identical extractions of one 4.3 GB file
overlapping, each holding a transcode slot and reading the file end to end.
Latecomers now wait on the answer the first is producing.

The in-flight entry is registered *before* the first await, not after. The
first version registered it after the probe, two concurrent requests both got
past the check while neither had registered, and the test for it failed —
which is the only reason this note can be written from the right side.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UGY17EPph5LsLzePPXhUVc

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