fix(workflows): execute email steps from template relation

[vladbisceanu]

Outcome

Valid API-created SEND_EMAIL steps now execute from their relational template binding. Recipient-only config no longer fails runtime validation, and a stale duplicated JSON templateId cannot override the relation.

Closes #18.

Paired upstream PR: useplunk#472

Proof

• Added integration coverage for the production API step shape and conflicting legacy JSON ID.
• Full build passed all 10 tasks.
• Functional suite passed: 43 files, 1,155 tests.
• Lint passed with existing warnings only.
• Earlier full run passed all 1,191 functional tests; two unrelated performance checks were noisy under contention. The isolated template throughput check passed 8/8; the existing 10k pagination fixture still times out while emitting its own event foreign-key errors.

Production follow-through

After merge, deploy one immutable image digest to API, worker, and dashboard; read back all three pins; then rerun the synthetic opt-in against the unchanged live workflow.