feat: expose playlist search surfaces

[Priveetee]

Summary

• add playlist results to search responses
• add channel playlists endpoint
• document the new response schemas in OpenAPI

Tests

• JAVA_HOME=/usr/lib/jvm/java-25-openjdk PATH=/usr/lib/jvm/java-25-openjdk/bin:$PATH ./gradlew test --tests dev.typetype.server.SearchRoutesTest --tests dev.typetype.server.ChannelPlaylistsRoutesTest validateOpenApi --no-daemon
• JAVA_HOME=/usr/lib/jvm/java-25-openjdk PATH=/usr/lib/jvm/java-25-openjdk/bin:$PATH ./gradlew compileKotlin --no-daemon
• git diff --check
• JAVA_HOME=/usr/lib/jvm/java-25-openjdk PATH=/usr/lib/jvm/java-25-openjdk/bin:$PATH ./gradlew clean check shadowJar --no-daemon
• local runtime curl /search and /channel/playlists