Parallelize cold rollout compression

[jif-oai]

Why

#25089 added the background worker for compressing cold archived rollouts, but the worker still processed files effectively one at a time: each compression job was sent to spawn_blocking and then awaited before the next file started. On machines with a backlog of archived rollouts, that makes catch-up slower than it needs to be even though the actual compression work already runs off the async runtime.

What Changed

• Queue rollout compression work in a JoinSet while directory traversal continues.
• Cap the worker at two in-flight compression jobs so it can overlap compression without turning the background task into unbounded blocking work.
• Drain pending jobs before returning, including the read_dir.next_entry() error path, so every launched job still contributes to the final compressed, skipped, and failed stats.
• Treat task join failures the same way as compression failures in the worker's warning and failure accounting.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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