#115 feat: add broadcast duplicate endpoint

Adds POST /broadcasts/{id}/duplicate as broadcasts/duplicate, next to cancel. The operation answers 201 with { object, id }, the shape the public-api handler sends in resend/resend-monorepo#9608. The docs page comes with the rollout's docs PR, tracked under DEV-2089.

yq eval . resend.yaml parses. Redocly reports the same 10 errors as main; the one new warning is the "no 4XX response" that the sibling templates/duplicate operation carries too.

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