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

[pixels26]

Close #943

Summary

Replaces the stub AnchorRegistry::create_payout with a real async HTTP client that POSTs payout requests to the configured Stellar Anchor API endpoint.

Changes

• config.rs: Added anchor_api_url field loaded from ANCHOR_API_URL env var (defaults to http://localhost:8081)
• stellar_anchor.rs: Refactored AnchorRegistry to hold a reqwest::Client and anchor base URL; create_payout now makes an async POST to {anchor_api_url}/transactions/send with the payout payload and parses the response into an AnchorPayout with proper status tracking (Pending/Processing/Completed/Failed)
• main.rs: Passes config.anchor_api_url to AnchorRegistry::new
• .env.example: Added ANCHOR_API_URL documentation
• api.rs: Updated call site to .await the now-async create_payout
• Tests: Updated AnchorRegistry::new() calls to pass an API URL

Anchor API Contract

The implementation follows SEP-31 conventions. The anchor is expected to expose a POST /transactions/send endpoint accepting:

{
  "beneficiary_address": "...",
  "beneficiary_name": "...",
  "token": "...",
  "token_amount": 0.0,
  "fiat_currency": "USD",
  "bank_name": "...",
  "account_number": "..."
}

And responding with:

{
  "id": "tx-id",
  "transaction_id": "alt-id",
  "status": "pending|processing|completed|failed",
  "exchange_rate": 1.0,
  "fiat_amount": 100.0,
  "fee": 0.5
}

[pixels26]

@ONEONUORA please trigger cli

[ONEONUORA]

@pixels26
Run cargo fmt --all -- --check

[pixels26]

@ONEONUORA please allow auto trigger

[ONEONUORA]

@pixels26
Run cargo clippy --all-targets --all-features -- -D warnings

[pixels26]

@ONEONUORA all solved

[ONEONUORA]

@pixels26
Great job