Remix a published showcase entry

Record that the authenticated caller has built on a published entry, and pay its author a credit grant from the existing credit ledger. **A bearer token is required**, and not only for the usual reasons: this call *moves credits* to another account. An unauthenticated POST that pays someone is a payout endpoint, so the loop is authenticated end to end. It succeeds only when the entry is published, its author has opted in as a remix template, and the caller is not the author. Every refusal answers 409 and **records nothing**: no remix row, no counter movement, no credit. That is what stops the public "remixed N times" number from being inflatable by anyone who can POST. How much the author earns is **configuration, not a constant** (`GROWTH_REMIX_AUTHOR_CREDITS`, in credits where 1 credit = $0.01), because the plan fixes no amount — it only says the author "earns credits". At the default of 0 the remix is still recorded and the author still sees who built on their work, but `creditsGranted` is `null` and no credits move, so the loop is wired without a number nobody agreed to. Set the variable and the same call pays. The record is written before the credit is granted. If the grant then fails, the remix stands (it happened) with its credit unmade and visible, which is recoverable; the other order could pay for a remix that never recorded, which is not.

curl -X POST "https://example.com/showcase/string/remix"

{  "remix": {    "id": "string",    "entryId": "string",    "authorId": "string",    "remixerId": "string",    "creditsGranted": 0,    "createdAt": "string"  },  "credited": 0}