feat(hub): add delete account cascade from Settings

[Tam-Leal]

Summary

• Add DELETE /api/auth/account with CSRF and email confirmation; cascade-delete snapshots, machines, API keys, audit rows, and the user.
• Invalidate all Hub sessions and clear cookies after deletion.
• Settings danger zone UI: type account email to enable delete; Pro users see billing cancel guidance (Lemon / support).

Test plan

• Settings shows danger zone; delete button stays disabled until email matches
• Wrong email → 400; missing CSRF → 403
• Successful delete logs out and login with same credentials fails
• After delete, keys/machines/snapshots for that user are gone from Hub DB
• Pro copy warns that Lemon subscription is not auto-cancelled
• Existing revoke key / remove machine flows still work