refactor(email): add explicit return types to email send functions (3 of 4)

[jeanduplessis]

Summary

PR series: 3 of 4 — merge in order: #1559 oxlint → #1560 CI → this PR → #1562 trpc
(This PR is independent of #1559 and #1560 and can merge in parallel with them.)

Add explicit return type annotations to all email send functions for tsgo 7.0 compatibility:

• tsgo 7.0 enforces TS2883, which requires explicit return types when the inferred type references non-portable vendor types
• Wrapper functions (e.g. sendOrgSubscriptionEmail) now declare Promise<SendResult> instead of relying on inference from send()
• sendViaMailgun now returns Promise<boolean> (true = sent, false = not configured) instead of leaking the mailgun client response type

Verification

• scripts/typecheck-all.sh --changes-only — pass
• pnpm format:check — pass

Visual Changes

N/A

Reviewer Notes

• This is a prerequisite for refactor(trpc): migrate declaration emit to tsgo and stop committing dist #1562 which switches the packages/trpc build from tsc to tsgo. Without explicit return types, tsgo fails to emit declarations for the email functions.
• The SendResult type ({ sent: true } | { sent: false; reason: ... }) was introduced by the NeverBounce PR. The wrapper functions previously had no explicit return type, so they inferred the mailgun/CIO vendor types which tsgo rejects.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• src/lib/email-mailgun.ts
• src/lib/email.ts

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_{Reviewed by gpt-5.4-20260305 · 1,011,248 tokens}