refactor(email): give the transactional sender resolver one home

[viktormarinho]

Follow-up cleanup on #7159. pr-ready-email.ts shipped a byte-for-byte copy of dbos-digest.ts's resolveSender — same body, same comment about reusing the invitation provider. My duplication; extracting it now, before a third caller copies it again.

The resolver moves next to createEmailSender, which is what it builds.

No behavior change. Both call sites resolve the same provider in the same order (explicit inviteEmailProviderId, else the first configured one), so a deployment with no provider still sends nothing and stamps nothing.

The workflow-source snapshot moves because dbos-digest.ts changed. Recovery-compatible — one call swapped for an identical one, no step added, removed or reordered — so DBOS_WORKFLOW_VERSION stays at 11.

Validation: workspace type checks, lint (14 pre-existing warnings, 0 errors), knip, formatting, and 8,709 unit tests all pass.

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Summary by cubic

Consolidates the duplicated transactional sender resolution from dbos-digest.ts and pr-ready-email.ts into one resolveTransactionalSender in email-providers.ts. No behavior change — deployments without a provider still send nothing and stamp nothing.

Refactors

• Moves the resolver next to createEmailSender, which it builds.
• Both call sites resolve the same provider in the same order.
• The workflow-source snapshot updates because dbos-digest.ts changed; recovery-compatible, so DBOS_WORKFLOW_VERSION stays at 11.

^{Written for commit cfa01e6. Summary will update on new commits.}