Validate payment card numbers: Luhn checksum plus network detection (Visa, Mastercard, Amex, Discover, Diners, JCB, UnionPay, Maestro) with per-network length rules. Returns the network, Luhn result, and last four digits. Structure only — never that the card exists or has funds. No card data is stored or logged. POST a JSON body like: {"card":"4111 1111 1111 1111"}

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