Draft: Add Ci::Builds.playable scope

What does this MR do and why?

This MR is a proof of concept for turning the Ruby-heavy Ci::Build#playable? method into a scope. Original discussion in !136816 (comment 1789493923)

Rather than needing to query all Builds in a relation, instantiate them, and then run the methods, we can run playable.any? to offload all the logic of returning a boolean into a much more efficient SQL query.

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