feat: expose channel search results

[Priveetee]

Summary

• add channels to SearchPageResponse
• map PipePipe ChannelInfoItem search results
• filter search response buckets according to selected contentFilter
• bump public search cache key to avoid serving mixed cached results

Runtime verification

• YouTube channels: items 0, playlists 0, channels 2
• YouTube playlists: items 0, playlists 2, channels 0
• YouTube Music artists: items 0, playlists 0, channels 20
• BiliBili channels: items 0, playlists 0, channels 20
• NicoNico playlists: items 0, playlists 10, channels 0

Tests

• JAVA_HOME=/usr/lib/jvm/java-25-openjdk PATH=/usr/lib/jvm/java-25-openjdk/bin:/home/ark/.local/bin:/home/ark/.bun/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl ./gradlew clean check shadowJar --no-daemon