Refactor Playwright to singleton browser with tab-based crawling

- Add PlaywrightBrowserService: singleton Chromium browser with auto-recovery
- Refactor WebCrawlerService/YouTubeTranscriptService to use shared browser tabs
- Fix YouTube transcript: extract from DOM panel + fmt=json3 fallback
- Keep browser window alive (about:blank instead of page.close)
- Add docs: X Window setup, operation manual, crawling guide

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

start-backend.sh

@@ -11,11 +11,8 @@ export PLAYWRIGHT_BROWSERS_PATH=/home/opc/.cache/ms-playwright
 export PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1
 
 # Xvfb virtual display for Playwright head mode (YouTube transcript)
-export DISPLAY=:99
-if ! pgrep -x Xvfb > /dev/null; then
-    Xvfb :99 -screen 0 1280x720x24 -nolisten tcp &
-    sleep 1
-fi
+# :99 = Xvfb (백그라운드), :1 = VNC (디버깅용)
+export DISPLAY=:1
 
 # Playwright driver-bundle requires exploded classpath (fat JAR extraction fails)
 BACKEND_DIR=/home/opc/sundol/sundol-backend