fix: stop hardcoding JWKS_KID dev-main for managed scaffolds

[Bccorb]

Problem

seamless init hardcodes JWKS_KID to "dev-main" in the generated scaffold (src/commands/init.ts:50, printed at init.ts:409). Managed instances pin their signing key id in Terraform: paidkey1 for paid tenants and trialkey1 for trials (seamless-terraform-service paid-jobs/main.tf and trial-jobs/main.tf). Nothing is read from the portal for it.

Scope

• Verify whether the scaffolded backend actually validates against the managed instance's JWKS kid, and if so, stop hardcoding dev-main for managed scaffolds.
• Either resolve the kid from the instance's JWKS endpoint at init time, or have the portal API expose it alongside the application payload.
• --local scaffolds can keep the dev default.

Why

A hardcoded dev kid against a managed instance whose kid is paidkey1 breaks token verification in the scaffold, which is exactly the zero-to-working-local-dev path the CLI exists to make fast.