perf(images): optimize high-traffic training assets

[carlospolop]

Problem

CloudFront logs attributed 13.371 GB (25.5% of a deduplicated two-day sample) to the 1,326,124-byte summer banner across the Wiki traffic sample. The most frequent promotional images had no long-lived cache metadata.

Changes

• Add versioned WebP assets and update the banner/discount references.
• Summer image: 1,326,124 → 123,344 bytes (90.7% smaller); the sampled 13.371 GB would become ~1.244 GB at the same request count.
• Certification badges: 99.3–99.6% smaller at 4× their 24px display height.
• Publish only these versioned files with Cache-Control: public,max-age=31536000,immutable and Content-Type: image/webp.
• Cover both English and translated deployments, including translation branches that do not yet contain the new theme/assets.
• Keep all old PNG URLs to avoid breaking cached/older pages.

Validation

• Visually compared PNG/WebP outputs; transparency, text, colors, and layout remain intact.
• Upload helper exercised with a fake AWS CLI: four correct uploads per prefix.
• Workflow YAML parsed; shell and JS syntax checks passed; git diff --check passed.

Created by Hermes after the AWS Wiki cost audit.

2026-08-04 safety re-review

The immutable upload helper now checks ETag, Cache-Control, and Content-Type before writing. Unchanged correctly-tagged assets incur only a HEAD request and are not uploaded again; a missing object or metadata mismatch is repaired. Mocked skip/repair tests, shell syntax, JavaScript syntax, workflow YAML parsing, git diff --check, and a full mdbook build all pass.