video_downloader: optional cookies_file input + enable node JS runtime

[MoonCaves]

Summary

youtube downloads via video_downloader currently fail on a lot of boxes — youtube now serves "Sign in to confirm you're not a bot" to anonymous requests (datacenter IPs get it consistently, residential increasingly too), and yt-dlp's n-challenge solving needs a JS runtime that isn't enabled by default when only node is installed. this PR adds an optional cookies_file input (netscape-format cookies.txt, passed through as yt-dlp's cookiefile) and enables node alongside deno in js_runtimes so the EJS challenge solver works on node-only machines. to be precise about the second part: the node runtime is enabled unconditionally (not only when cookies are passed) — yt-dlp's embedded API documents js_runtimes as a dict of runtimes to enable, defaulting to {'deno': {}} (yt_dlp/YoutubeDL.py lines 539–545 and 736), so on machines that have deno nothing changes, and on node-only machines the solver starts working instead of failing.

also adds /cookies.txt (anchored to repo root) to .gitignore so nobody accidentally commits their session cookies to a fork.

Related issue

Refs # (none found — happy to open one first if you prefer)

Changes

• tools/analysis/video_downloader.py: new optional cookies_file input on the schema, threaded through metadata extraction, video/audio/subtitle download paths via a shared _common_ydl_opts; js_runtimes now {"deno": {}, "node": {}}
• .gitignore: ignore /cookies.txt at repo root

Testing

• on a debian 12 server (datacenter IP, node 22, no deno): without cookies, downloads fail with youtube's bot-check; with a cookies.txt exported from a logged-in browser, metadata + video + audio + subtitles all succeed end-to-end
• same box: without node in js_runtimes, formats fail with "n challenge solving failed" / "No video formats found"; with it, formats resolve

Checklist

• The change is focused on a single logical concern.
• I ran the relevant tests locally (make test-contracts / make test) where applicable. (contract suite on a clean checkout of main with only this change applied: 325 passed, 6 skipped)
• I updated docs/README if behavior or usage changed. (the tool's schema description documents the new input; happy to add a README note if wanted)
• No unrelated files (build artifacts, local config) are included in the diff.