api/youtube: reuse web visitor data for android vr

[Rexarrior]

summary

• bootstrap anonymous visitor data from the public YouTube watch page when using ANDROID_VR
• cache it briefly and refresh the Innertube session when it changes
• send the matching Android VR user agent and treat its direct media URLs as no-cipher

why

YouTube currently ties playable Android VR player responses to visitor data issued by the watch page. youtubei.js otherwise creates an unrelated visitor session, which returns LOGIN_REQUIRED; the default iOS flow may produce tunnels whose Google Video responses are empty.

This is opt-in through CUSTOM_INNERTUBE_CLIENT=ANDROID_VR.

Addresses #1455 and its duplicate #1475.

testing

• node --check api/src/processing/services/youtube.js
• git diff --check
• built and ran the Docker image with CUSTOM_INNERTUBE_CLIENT=ANDROID_VR
• requested https://youtu.be/c5cb0pqky3s?list=RDc5cb0pqky3s; the API returned a tunnel
• read 1 MiB from the tunnel successfully (previously 0 bytes)