feat(cli): configure sourcemap upload concurrency

[DebadityaHait]

Problem

Closes #70238

Sourcemap S3 uploads use a process-wide Rayon pool hardcoded to 10 threads, so large builds cannot tune upload concurrency.

Changes

• Add --concurrency and POSTHOG_CLI_SOURCEMAP_UPLOAD_CONCURRENCY to sourcemap upload and sourcemap process.
• Replace the global Rayon pool with a local symbol-set upload pool.
• Keep 10 as the default for all existing upload paths, and reject zero or malformed values.

How did you test this code?

• cargo +1.91.1 test concurrency --lib – 9 concurrency tests pass.
• cargo +1.91.1 test --all-features -- --skip test_pair_inject – 136 tests pass. On this Windows checkout, the skipped fixture test has the same CRLF/LF mismatch on unmodified master.
• cargo +1.91.1 clippy --all-targets --all-features -- -D warnings -A clippy::needless-borrows-for-generic-args – clean; the allowed lint is an unchanged Windows-only finding in login.rs.
• cargo +1.91.1 fmt --all -- --check, release build, and Rustdoc with warnings denied – clean.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

Updated cli/README.md with the default, precedence, direct CLI usage, and build-environment usage for @posthog/nextjs-config.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Codex made the changes and ran the checks using the public contribute skill. I chose a PostHog-specific setting and a local pool so sourcemap builds can tune their upload workload without changing unrelated global Rayon behavior.

[hpouillot]

Thanks for your contribution @DebadityaHait !

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-07-15 10:50 UTC	Run
prod-us	✅ Deployed	2026-07-15 11:03 UTC	Run
prod-eu	✅ Deployed	2026-07-15 11:09 UTC	Run