Serve single-video galleries through video API

[zachlatta]

What changed

• project complete, single-video gallery bundles onto Arker's normal video manifest and download contract
• expose their stored raw provider record through /video/:shortid/raw
• preserve photo, carousel, partial, and ambiguous legacy captures as galleries
• reuse the gallery media serving path so HEAD, Range, MIME sniffing, and size limits remain consistent

Why

Instagram serves the same Reel at both /p/{shortcode} and /reel/{shortcode}. Arker must choose an extractor from the submitted path before it can see the media type, so a Reel submitted through /p/ can be correctly captured as one MP4 inside a gallery bundle but remain invisible to video consumers.

This makes the stored artifact's proven shape authoritative at response time without duplicating downloads or changing gallery routing.

Validation

• go test ./... -count=1
• go vet ./...