fix: include compute_tier in create-app payload (#64)

## Summary
- `ToPayload()` mapped every scaling field except `ComputeTier`, so
`compute_tier = "protected"` in `cerebrium.toml` was silently dropped
from the API request body
- Companion backend PR: CerebriumAI/dashboard-backend —
`jono/fix-compute-tier-in-create-app`

## Test plan
- [ ] Set `compute_tier = "protected"` in cerebrium.toml and deploy —
verify `computeTier` appears in the request body
- [ ] Deploy without `compute_tier` set — verify no regression

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jonoirwinrsa

pkg/projectconfig/config.go

@@ -160,6 +160,9 @@ func (pc *ProjectConfig) ToPayload() map[string]any {
 	if pc.Scaling.LoadBalancingAlgorithm != nil {
 		payload["loadBalancingAlgorithm"] = *pc.Scaling.LoadBalancingAlgorithm
 	}
+	if pc.Scaling.ComputeTier != nil {
+		payload["computeTier"] = *pc.Scaling.ComputeTier
+	}
 
 	// Runtime configuration
 	if pc.CustomRuntime != nil && pc.PartnerService != nil {