Merge feature/RECIPE-0009_deduplication_notifications_ui

RECIPE-0009: deduplication, notifications, UI improvements
- Iteration 0: deduplication, push notification subscribe, UI
- Iteration 1: footer status bar, icon-only buttons
- Iteration 2: ARIA-compliant footer icon contrast

yt-dlp extractor:
- Replace Playwright scraper with yt-dlp subprocess
- Feature flag EXTRACTOR_BACKEND (ytdlp|playwright)
- Dockerfile: add yt-dlp via pip3

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitignore

@@ -18,6 +18,9 @@ Thumbs.db
 !.env.example
 !.env.test
 
+# Secrets (never commit cookies, tokens or credentials)
+secrets/
+
 # Local certificates
 .ssl/