#1059 feat: add broadcasts.cancel() method

Summary

• Adds broadcasts.cancel(id), calling POST /broadcasts/:id/cancel to cancel a queued or scheduled broadcast.
• Returns a validation_error/403 if the broadcast is draft, sent, or failed.
• Mirrors emails.cancel()'s shape exactly (no body, no options param) since the underlying endpoint is the same kind of simple id-only action.

Companion API docs: resend/resend-docs#(branch broadcasts-cancel, based on main).

Test plan

• pnpm typecheck — clean
• lint (scoped to touched files — the full repo-wide lint script chokes on committed .next build artifacts under e2e/, unrelated to this change)
• pnpm test — 28/28 passing in broadcasts.spec.ts (26 existing + 2 new: success case, 403 not-cancelable case)

🤖 Generated with Claude Code

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Summary by cubic

Add broadcasts.cancel(id) to cancel queued or scheduled broadcasts via POST /broadcasts/:id/cancel, matching emails.cancel().

• New Features
  • Cancels by ID with no body/options; returns validation_error (403) if the broadcast is draft, sent, or failed.
  • Adds CancelBroadcastResponse types and exports under src/broadcasts/interfaces.