🧹 [Code Health] Centralize duplicated download_video code into common utility

[Jandir]

🎯 What: The code health issue addressed is the massively duplicated logic for yt-dlp download_video orchestrator in vimeo.py and youtube.py.
💡 Why: Duplicated logic spanning over hundreds of lines of code is prone to bugs and makes updates harder. Centralizing extraction flow, smart language mapping, progress hooks, HTTP 429 auto-healing, and cookie refreshes into a single utility file (dl_common.py) significantly streamlines and declutters both platform-specific modules. This leads to cleaner, drier, and more maintainable architecture.
✅ Verification: Verified by ensuring the exact behavior, argument mapping, and edge-case behaviors mapped identically to the previous version and confirming that pytest tests/ successfully passed all 92 unit tests in less than three seconds.
✨ Result: Both vimeo.py and youtube.py now feature a lightweight wrapper around the core downloader logic, preserving functionality while discarding over 400+ redundant lines of boilerplate code.

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PR created automatically by Jules for task 2635602496290024089 started by @Jandir

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