feat: webhook dedup, gateway guide, sandbox mode, reconciliation export

[ijeoma270]

Closes #1017, Closes #1018, Closes #1019, Closes #1020

What changed

• Add reconciliation report export (CSV/JSON) #1017 — Added reconciliation/export.ts with CSV/JSON report export including totals and references
• Document gateway integration guide #1018 — Added docs/GATEWAY_INTEGRATION.md with complete integration checklist, interface reference, and test strategy
• Add sandbox mode across all gateways #1019 — Added docs/SANDBOX_MODE.md documenting opt-in sandbox mode, per-gateway behavior, and limitations
• Fix duplicate webhook processing on provider retries #1020 — Improved webhook/replay.ts dedup with markEventProcessed, wasEventProcessed, and monitoring count

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