fix: stop labeling bot-check videos as members-only

[Priveetee]

Summary

• keep real members-only prompts mapped to the existing members-only message
• map YouTube bot-check/sign-in verification prompts to a dedicated english sign-in verification message
• add sanitizer tests for bot-check, sign-in, and members-only paywall prompts

Validation

• JAVA_HOME=/home/ark/.jdks/jdk-17.0.18+8 ./gradlew test
• yt-dlp check confirms several reported IDs are public while backend returned 422 members-only before this fix