fix(media): import, upload, and serve AVIF images

[DavidBabinec]

Fixes #408

The bug

AVIF files were filtered out of static imports, misidentified as MP4 during upload sniffing, and served as generic downloads. A reported Webflow import silently lost 353 images.

The fix

Accept AVIF across the import and upload policies, detect its ftyp/avif brand before the broader MP4 signature, generate the existing responsive image ladder, and serve it as image/avif. Regression coverage pins each boundary and the media/import docs now name the supported format.

Verification

production fix removed; targeted tests  # 40 pass, 3 fail
bun test <three regression files>        # 43 pass, 0 fail
bun run build                            # clean, exit 0
bun run lint                             # clean, exit 0
bun test                                 # 6727 pass, 0 fail, exit 0