Remove mod_deflate and ETag stripping to fix CDN 404s

[davsclaus]

Summary

• Remove the entire mod_deflate compression section (~200 lines) from .htaccess
• Remove the ETag stripping section

Why

The sporadic 404 errors (#1631, #1565, INFRA-27679) are correlated with Accept-Encoding: gzip — browsers get 404s but curl (which doesn't send gzip by default) works fine.

mod_deflate removal: The .htaccess had Apache's mod_deflate handling compression on the origin, including a decade-old workaround for "mangled Accept-Encoding" headers. With Fastly CDN in front, the CDN handles compression — having Apache also do it can interfere with Fastly's Vary: Accept-Encoding cache key handling, contributing to the gzip-specific 404s. The removed sections include:

• SetEnvIfNoCase workaround for mangled encoding headers
• AddOutputFilterByType DEFLATE for all MIME types
• AddEncoding gzip svgz
• Commented-out Brotli and GZip pre-compressed content sections (unused)

ETag removal: The .htaccess was stripping ETags (Header unset ETag + FileETag None). ETags are actually useful for Fastly's cache validation — removing them forces full re-fetches on every cache miss instead of efficient 304 Not Modified responses.

Test plan

• Verify pages still load correctly after deploy
• Verify Content-Encoding: gzip is still present in responses (handled by Fastly)
• Monitor for 404 recurrence, especially with Accept-Encoding: gzip requests
• Verify .svgz files still render correctly (Fastly should handle the encoding)

Fixes #1631