Replace MinIO with SeaweedFS for result storage

[soulgalore]

MinIO's community edition is dead — the GitHub repo was archived in
April 2026 and the last free release (October 2025) gets no further
security patches, which is untenable for the service that publicly
serves test results (#215). SeaweedFS is Apache-2 licensed, actively
maintained, ships official multi-arch images and covers everything we
used MinIO for: S3 uploads from the testrunner, anonymous public reads
of results, and a 30 day expiry.

The swap is interface-compatible to keep the change small: the S3 API
stays on port 9000, the MINIO_USER/MINIO_PASSWORD variable names are
kept, and result URLs don't change, so an existing deployment migrates
by updating two lines in .env (the image pin and the endpoint
hostname). PRODUCTION.md gets an upgrade guide covering all three
deployment shapes. The expiry uses SeaweedFS native TTL (fs.configure)
instead of an S3 lifecycle rule, because lifecycle rules have a
history of not being enforced. The anonymous identity is read-only and
scoped to the result bucket, matching MinIO's "anonymous set
download". MinIO's storage compression for html/css/js has no
equivalent and is dropped — it only cost disk, never bandwidth.

Old results are not migrated to the new data volume — they would have
expired within 30 days anyway. Verified end-to-end locally: a real
sitespeed.io run uploaded its full result folder through the s3 plugin
and the report serves back anonymously with correct content types.

Co-authored-by: Claude Fable 5 noreply@anthropic.com