Adds lib/audit-coverage.py: queries Jellyfin live for every series, every
episode, and every movie; classifies each by whether the English subtitle
comes from a sidecar, embedded stream, or doesn't exist; renders a
Markdown report with one-char-per-episode bars for visual scanning. Output
file is processes/subtitles/COVERAGE.md, regenerated on demand.
v2 sub-rest-fetch.py and v3 sub-a7d-fetch.py now invoke the audit at end
of a successful run, so the committed coverage file stays in sync with
library state without manual intervention. v3.5 yt-fetch path skips the
auto-call since it doesn't speak to Jellyfin directly; run audit manually
after copying YT sidecars to nullstone.
README.md surfaces the audit at the top so anyone landing in the recipe
folder sees current state before starting a run.
