feat: trigger fiat payout to Stellar Anchors (#943)

[pixels26]

Summary

Trigger fiat payout to Stellar Anchors** by replacing the stub AnchorRegistry with real HTTP calls to Stellar Anchor API endpoints.
Close #943

Changes

Backend — Anchor API Integration (backend/src/stellar_anchor.rs)

• AnchorRegistry::create_payout() — sends POST {ANCHOR_API_URL}/transactions with SEP-31-style payload (amount, asset_code, destination_asset, beneficiary bank details). Parses response for exchange rate, fiat amount, and anchor fee. Falls back to simulated payout when API is unconfigured.
• AnchorRegistry::get_payout() — sends GET {ANCHOR_API_URL}/transactions/{id} to retrieve payout status.
• AnchorRegistry::list_payouts() — sends GET {ANCHOR_API_URL}/transactions with optional sender_id filter.
• Added AnchorError enum with Http, Api, NotConfigured, NotFound, InvalidResponse variants.
• Status mapping handles pending, processing/in_progress, completed/success, failed/error/rejected.

Configuration

• backend/src/config.rs — added anchor_api_url and anchor_api_key fields.
• backend/.env.example — added ANCHOR_API_URL and ANCHOR_API_KEY env vars.

Database Migration

• backend/migrations/20260726000000_add_anchor_payout_id.up.sql — adds nullable anchor_payout_id TEXT column to payouts table with index.
• backend/migrations/20260726000000_add_anchor_payout_id.down.sql — reverses the migration.

Payout Flow (backend/src/api.rs)

• trigger_payout handler now calls state.anchor.create_payout(req).await and stores the returned anchor payout ID in the payouts table.
• PayoutRow now includes anchor_payout_id: Option<String> in responses.
• get_anchor_payouts endpoint includes anchor_payout_id in results.

Tests & Setup

• main.rs, api_tests.rs, kyc_webhook_test.rs updated to pass config to new AnchorRegistry::new(api_url, api_key) constructor.

Usage

Set these environment variables to connect to a Stellar Anchor:

ANCHOR_API_URL=https://anchor.example.com/api
ANCHOR_API_KEY=your-api-key

When ANCHOR_API_URL is unset, the system falls back to simulated payouts (development mode).

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Run cargo fmt --all -- --check and clippy

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@ONEONUORA please check now, my apologies for the delay

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Run cargo fmt --all -- --check

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on it now

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please check @ONEONUORA

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Run cargo clippy --all-targets --all-features -- -D warnings

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@ONEONUORA please check now

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Run cargo clippy --all-targets --all-features -- -D warnings

[pixels26]

please check

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Run cargo clippy --all-targets --all-features -- -D warnings

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i'm fixing now

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Run cargo fmt --all -- --check
also run clippy