feat(oauth): support client_secret_basic for Aikido

[airbooks-alpha]

Summary

Pulls in the focused implementation from UsefulSoftwareCo#1448 so Executor can connect to OAuth providers such as Aikido that require client_secret_basic.

• persist the selected token-endpoint authentication method in PostgreSQL, SQLite, and Cloudflare D1
• support client_secret_post and client_secret_basic in client creation and UI
• preserve the method across authorization-code exchange, client-credentials minting, refresh, and re-minting
• retain client_secret_post as the default

Upstream source: UsefulSoftwareCo#1448

Verification

• changed-file formatting: passed
• lint: passed
• typecheck: 44/44 packages passed
• Cloudflare migration assertions passed
• OAuth tests reached their assertions but local Effect/Bun teardown exceeded the test timeout; upstream PR reports the complete focused suite green and a manual Aikido client-credentials connection

Scope

This is the upstream four-commit patch applied cleanly on top of our fork. No additional custom implementation was added.