Add YouTube subtitles, local transcription, self-updating yt-dlp/Deno, X Article and thread titles

- Capture YouTube subtitles by default (opt-out in UI, API, CLI --no-subtitles)
- Summarize YouTube videos from subtitles; fetch on demand, then local transcription, then error
- Local transcription fallback: Whisper, Parakeet, Phonon-2 (English only)
- Runtime-resolved, self-updating yt-dlp and Deno JS runtime (fixes YouTube 403s)
- Settings > Instance > yt-dlp: status and in-app update without restart
- X Article titles from article.title, with idempotent startup backfill
- Thread title generation (single and bulk) with per-provider cheap models
- Per-provider title model settings in Settings > Instance
- Docs, mental model, AGENTS.md and transcription spec updated

crates/archivr-core/Cargo.toml

@@ -15,6 +15,10 @@ sha3.workspace = true
 uuid.workspace = true
 reqwest = { workspace = true }
 base64.workspace = true
+zip.workspace = true
+
+[target.'cfg(unix)'.dependencies]
+libc.workspace = true
 
 [dev-dependencies]
 tempfile = "3"