perf: make Hacktoberfest prep tracker fast + verbose (async fan-out, progress)

[priya-sundaram-dev]

Follow-up to #15225, addressing @cclauss's feedback on #15081: the daily hacktoberfest_prep run took ~16 minutes with no output.

What was slow

scripts/hacktoberfest_prep_update.py looks up the changed files of every open awaiting reviews PR to rank the busiest directories — up to a few hundred GET /pulls/{n}/files calls, previously issued one at a time. That serial chain, not the total request count, is what ate the 16 minutes.

Changes

• Async fan-out. Those lookups are independent, so they now run concurrently through a single httpx2.AsyncClient (per the async guide) bounded by a small asyncio.Semaphore (CONCURRENCY = 8) to stay well under GitHub's secondary rate limits. Tracked-row PR-state checks and the three issue/PR counts are gathered concurrently too. Runtime drops from ~16 min of serial round-trips to well under a minute.
• Progress reporting. Each phase logs to stderr (flushed, so it streams live in the Actions log): row-resolution, count collection, and a ...scanned N/total tick every 25 PRs. The final line now reports elapsed seconds.
• File I/O stays in the sync main(); only the network work is async (keeps ASYNC230 happy).

Rate-limit/5xx retry behaviour is unchanged, just moved into the async helper. Stdlib asyncio + the already-installed httpx2, so no workflow change is needed.

On the two links you shared

• httpx2 async — yes, this is the win, applied above.
• pydantic-ai eval retry strategies — that one is for pydantic-ai evaluation runs (retrying flaky LLM eval cases), so it doesn't map onto a plain GitHub REST poller. The reliability lever here is the concurrency cap + the existing X-RateLimit-Reset/5xx backoff, which I kept.

Testing

Ran the whole flow against a stubbed client (checkbox ticking for merged/closed vs still-open rows, stats block, top-3 directory ranking with correct singular/plural, progress lines, exit code). ruff check/ruff format clean.

[algorithms-keeper]

Closing this pull request as invalid

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[cclauss]

RSLGTM