3rd Party Payment Gateway integration

[mrojz]

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This PR fixes issue #1774.

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[jmanico]

Fantastic work. Here are some AI driven suggestions from ChatGPT 5 to consider before we finalize this.

Fix wording:

• “PCI compliance out-of-the-box” → “Reduced PCI DSS scope if integrated correctly.”
• “The following sequence diagram explain” → “explains.”
• Fix “3rd Pary” → “3rd Party.”

Add a line clarifying user redirection params are untrusted, only server callbacks count.

Also:

• Add fraud/risk scoring mention to cover carding attacks.
• Add transaction ID + expiry to replay mitigation.