feat(db): support runtime migration fallback for Hyperdrive-only configs

[onmax]

Summary

PR #868 fixes the build failure for Cloudflare Hyperdrive-only PostgreSQL/MySQL configs by skipping build-time migrations when no direct build-time connection URL is available.

This follow-up tracks the missing feature: supporting automatic migrations for deployments that rely on connection.hyperdriveId alone.

Related context

• fix(db): lazy URL resolution for Docker/K8s deployments #790 introduced lazy runtime DB resolution for non-Cloudflare deployments. That pattern is the closest prior art for a runtime fallback here.
• 🐞 NuxtHub does not automatically apply D1 migrations when deploying via Cloudflare Git integration #861 and Migrations not applied during GH Actions build for CF deployments #725 established that Cloudflare migration gaps are often deployment-workflow issues, not something wrangler deploy handles automatically.