docs: sync from backend 3a50dc4 - promote postgres integration

PRD - Gasless Buyer Payments (Roadmap).md

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+# PRD — Gasless Buyer Payments (Roadmap)
+
+Status: **Roadmap / future improvement** for full gasless payments. The partial permit-approval relay shipped on backend `integrate-main-into-development` at `3a50dc4`.
+
+## Background
+
+The in-house checkout (Request Network fee-proxy + amn.scanner) has the buyer:
+1. **approve** the RN fee-proxy to spend their token (on-chain tx, gas), then
+2. **pay** via `transferFromWithReferenceAndFee` (on-chain tx, gas).
+
+We want the buyer to pay **gasless** (sign only, never spend native gas) when the
+token supports it.
+
+## Partial (shipped — permit-approval only)
+
+For EIP-2612 permit-capable tokens (USDC on mainnet/Base/Arbitrum/Polygon — see
+`PERMIT_CAPABLE_TOKENS` in `sweepService.ts`; **mainnet USDT has NO permit**):
+- Buyer signs an EIP-2612 **permit** (gasless signature) granting allowance to the
+  fee-proxy.
+- A backend **relayer** broadcasts `token.permit(...)` (relayer pays that gas).
+- Buyer still pays gas for the **transfer** (`transferFromWithReferenceAndFee`).
+
+Net: removes the *approve* tx gas only. USDC-only. The buyer still sends 1 tx.
+
+## Full gasless (THIS roadmap item — NOT done)
+
+**Blocker:** `transferFromWithReferenceAndFee` pulls tokens from **`msg.sender`**,
+so a relayer calling it would pull from the *relayer*, not the buyer. A relayer
+cannot broadcast the payment on the buyer's behalf with the current contract.
+
+To make the buyer fully gasless (sign only), build ONE of:
+
+1. **Meta-tx forwarder / custom payment proxy** — a contract that accepts a buyer
+   EIP-2612 permit + a signed payment intent, then `transferFrom(buyer, …)` while
+   the **relayer** is `msg.sender` and sponsors gas. Requires a deployed,
+   audited contract + funded relayer wallet + replay/abuse protection.
+2. **ERC-4337 account abstraction + paymaster** — buyer ops sponsored by a
+   paymaster. Requires bundler + paymaster funding + smart-account UX.
+
+### Requirements / open questions
+- Deployed contract (forwarder or AA stack) per supported chain.
+- Funded relayer/paymaster wallet; gas-cost accounting (who eats the gas, caps).
+- Abuse controls: bind each sponsored op to a real pending payment
+  (paymentId + buyer + spender + amount), rate-limit, deadline.
+- Non-permit tokens (mainnet USDT) can never be permit-gasless — needs AA or a
+  pre-funded-allowance flow.
+
+### Out of scope
+- The partial permit-approval flow (separate, smaller change).
+- Production relayer funding/ops.