feat: add trailer download script with shared library

[mvanbaak]

Summary

• Extract shared library scripts_common.sh from tag_dvfelmel.sh (config loading, executable checks, Radarr API helpers)
• Refactor tag_dvfelmel.sh to source the shared library, removing duplicated code
• Add download_trailer.sh - downloads official YouTube trailers from TMDB for Radarr movies
  • Supports MovieAdded, Download, and Bulk event types
  • Downloads trailers in movie's original language and configurable additional languages (default: pt-BR)
  • Downloads Brazilian Portuguese subtitles for original-language trailers as separate .srt files
  • Tracks downloaded trailers per-movie via yt-dlp --download-archive
  • Supports optional YouTube cookie authentication for age-restricted content
  • Creates movie/Trailers directories only when trailers are available
• Add docs/cookie-extraction.md guide for exporting YouTube cookies from browsers (Linux, macOS, Windows, Docker)
• Update scripts.conf.sample, README.md, and AGENTS.md with new config variables, commands, and dependencies

Files Changed

File	Change
radarr/connect/scripts_common.sh	New - shared library
radarr/connect/tag_dvfelmel.sh	Refactored to source shared library
radarr/connect/download_trailer.sh	New - trailer download script
radarr/connect/scripts.conf.sample	Added TMDB_API_KEY, TRAILER_*, YT_DLP_COOKIE_FILE
docs/cookie-extraction.md	New - cookie extraction guide
README.md	Added download_trailer.sh section
AGENTS.md	Added new files, commands, and dependencies

Test Plan

• shellcheck -e SC3043 passes on all three shell scripts (only expected SC1091 info)
• sh -n syntax check passes on all scripts
• ./tag_dvfelmel.sh test mode outputs success message
• ./download_trailer.sh test mode outputs success message
• download_trailer.sh exits with error on missing TMDB_API_KEY
• Manual test with real Radarr instance and TMDB API key
• Verify trailer download with cookie file for age-restricted content