docs: cover Echo v5.4.0 and v4.16.0 security behavior

[vishr]

Closes #449.

What changed

• Pin the published v5 reference and the example modules to Echo v5.4.0. Refresh the next pin to current Echo master so it no longer predates the release.
• Add a request scheme and trusted proxies guide. Update IP, redirect, HSTS, Proxy, JSONP, MethodOverride, Static, trailing slash, and testing guidance for the v5.4.0/v4.16.0 security behavior.
• Update Spanish, Japanese, Chinese, and Portuguese pages. Add a build gate that detects future English or translated edits to these 13 security pages until their translation baselines are reviewed again.
• Review the new routes and translation baselines.

Verification

• npm run build (stable and next builds, source checks, translation checks, 687 routes and 61,476 internal links/assets, size checks)
• go test ./... in the root and reference/ modules
• git diff --check and typos on the key English guide and script

The Echo repository's llms.txt and llms-full.txt list API names and examples but do not describe the changed security behavior, so this issue requires no change to those files.