[FEAT] Associate uploaded image as og:image for a shortened URL

[jo-chemla]

Use case

I'm building terrain-viewer, a pure frontend app that generates dynamic visual content (e.g. a terrain generator using canvas). When users want to share their current view to socials, they need a short URL whose link preview reflects a screenshot of the current state.

Zipline already does both pieces independently:

• POST /api/upload → returns a public image URL
• POST /api/shorten → creates a short URL

But there's no way to associate an uploaded image with a short URL so that the shortened link renders that image as its og:image in social previews.

Proposed change

Allow an optional ogImage field on POST /api/shorten, referencing either:

• A Zipline-hosted file URL (from a prior upload), or
• Any external URL
• bonus point if one can pass directly the base64 image, which would first upload to zipline storage, then adds as url og-media metadata.

When someone visits the short URL, Zipline renders an intermediate HTML page with correct og:image, og:title, og:description meta tags before the JS redirect, ensuring social crawlers pick up the custom preview.

Why this matters

Social crawlers (Twitter/X, LinkedIn, Facebook, Slack, Discord, WhatsApp) fetch OG metadata within seconds to minutes of a link being shared. This is a one-time crawl per URL — the image does not need to be stored permanently. Even a short TTL upload (e.g. 30 minutes) would be sufficient for crawlers.

Ideal one-call flow

POST /api/shorten
{
  "url": "https://myapp.com?params=xyz",
  "ogImage": "https://zipline.mydomain.com/u/terrain_preview.png",
  "ogTitle": "My Generated Terrain",
  "ogDescription": "Check out this terrain I generated!"
}
→ { "shortUrl": "https://zipline.mydomain.com/s/abc123" }

Alternatives considered

• Using separate services (imgbb + dub.co) — works but adds two API keys and two round trips to an otherwise self-hosted setup
• Cloudflare Workers — viable but adds infrastructure complexity