feat: automatic region placement for Ignite workloads

[t0gun]

Context

Ignite is stateless serverless compute. Users should not need to think about where it runs — they care that it is fast, not which region it landed in.

Behavior

• If region is omitted from the deploy request, the platform selects the best region automatically
• If region is specified, respect it (compliance, latency pinning)

Automatic placement strategy

When no region is specified, select based on (in priority order):

1. Closest to request origin — resolve the client's IP to a geographic region, auction there first
2. Available capacity — if the nearest region has no capacity, fall back to the next closest region with available nodes
3. Lowest auction response time — among tied candidates, the fastest bid wins (already how auctions work)

API

POST /v1/ignite
{
  "image": "registry.example.com/app:latest",
  "region": "ca-central"  // optional — omit for auto-placement
}

Error responses

• Region specified but invalid: 400 Bad Request — {"error": "unknown region: ca-east"}
• Region valid (or auto) but no capacity: 503 Service Unavailable — {"error": "no capacity available"}

Implementation notes

• Geo-IP resolution at the control plane (MaxMind GeoLite2 or Cloudflare headers if fronted by CF)
• Define a region priority list per geo bucket (e.g., Canadian IP → ca-central first, us-east fallback)
• Auction broadcast targets the selected region; on timeout, try next region in fallback list
• For now with a single region, auto-placement just defaults to the only available region