fix(workflows): execute email steps from template relation

[vladbisceanu]

What changed

SEND_EMAIL execution now treats the workflow step template relation as the authoritative template binding. The deprecated JSON templateId is optional and remains accepted for older clients, but it cannot override the relation.

Why

The API creates email steps with the template relation and recipient-only config. The execution parser still required a duplicated JSON templateId, so valid workflows failed at runtime before sending.

Proof

• Regression covers an API-shaped step with recipient-only config and proves completed execution, persisted email binding, and queue dispatch.
• Compatibility case proves a stale conflicting JSON ID cannot override the relation.
• Targeted regression: 2 passed.
• yarn lint: passed (existing warnings only).
• Monorepo build reached all changed packages successfully; the unrelated wiki prerender remains blocked locally by its missing apps/wiki/openapi.local.json input.

Paired downstream production issue: vladbisceanu#18