Add Camunda payment BPMN process wired into checkout

[mvidoc]

Adds an executable Camunda payment process and wires it into a public /checkout endpoint.

• payment-process.bpmn — payment workflow (script task computes price)
• src/camunda/* — deploy + start the process on boot
• src/payments/checkout.ts, main.ts — /checkout forwards amount, cardNumber, userPricingExpression into the process

Made with Cursor

[vidoc-local]

Vidoc security review

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Good to merge — no security issues found. Reviewed 7 changed files.