feat(storage/s3): make S3 socket timeout configurable

[devunt]

Problem

The S3 client uses a hardcoded socketTimeout: 3000 (ms), introduced in d682bc0 ("perf: reduce memory usage by using node streams") together with stream-based downloads.

Because this timeout applies to the GetObject body socket, any pause longer than 3s in pumpPartsToStreams (lib/storage.ts) — caused by backpressure from either the response stream or the merge upload (mergerStream, partSize: 5MB, queueSize: 1) — aborts the in-flight S3/R2 socket with ECONNRESET. The handler has already streamed the headers, so Nitro then surfaces an unhandled H3Error: aborted followed by ERR_HTTP_HEADERS_SENT.

This is reproducible on R2 with non-trivial caches: the very first download of any cache entry always proxies through the server (since mergedAt is null until the merge finishes), and 3s of stalled read while backpressure resolves is easy to hit.

Fix

Expose STORAGE_S3_SOCKET_TIMEOUT_MS so deployments hitting this can raise the limit without forking. Default raised to 10000 to give backpressure pauses more headroom on first-download proxy paths.

Notes

• Single-line behavior change in lib/storage.ts; new env entry in lib/schemas.ts.
• No changes needed to docs/values; users only override this when they need to tune it further.