feat(backend): implement BridgePayoutEvent webhook listener (#33)

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Closes #942

Summary

Adds a background daemon that polls the Stellar Horizon API for Soroban contract events with the BridgePay topic, persists them with exactly-once semantics, and fans out bridge.payout webhook dispatches to all active registered endpoints.

Changes

backend/src/bridge_event_listener.rs (new)

• BridgeListenerConfig::from_env() — reads STELLAR_CONTRACT_ID, STELLAR_HORIZON_URL, BRIDGE_LISTENER_POLL_INTERVAL_SECS; returns None when contract ID is absent so the daemon self-disables
• BridgeEventListenerService — poll loop with tokio::select! and a watch::Receiver shutdown channel for graceful termination
• run_once() — full poll → parse → persist → webhook cycle
• latest_persisted_ledger() — fixed fetch_optional → fetch_one (COALESCE always returns a row; old code skipped every poll cycle silently)
• fetch_events() — Horizon HTTP poll with correct 19-digit zero-padded cursor format {seq:019}-0000000000
• persist_event() — INSERT … ON CONFLICT DO NOTHING for exactly-once DB persistence keyed on (contract_id, ledger_sequence, tx_hash, event_index)
• enqueue_webhook() — fan-out via WebhookDispatcherService
• extract_str() / extract_i64() — handle plain strings and Horizon wrapper objects ({id:…}, {value:…})
• 23 inline unit tests

backend/src/api.rs

• GET /api/bridge/events — paginated query with contract_id, owner_address, from_ledger, to_ledger, page, page_size filters
• BridgeEventQuery, BridgeEventRow, BridgeEventsResponse types
• Route added to public_routes in create_router

backend/src/lib.rs

• pub mod bridge_event_listener
• pub use BridgeEventListenerService, BridgeListenerConfig

backend/src/main.rs

• Creates tokio::sync::watch shutdown channel
• Starts BridgeEventListenerService gated on STELLAR_CONTRACT_ID being set
• Graceful shutdown tied to Ctrl-C via axum::serve::with_graceful_shutdown

backend/migrations/20260724000000_create_bridge_payout_events.{up,down}.sql

• bridge_payout_events table with unique index for dedup
• Indexes on ledger_sequence for efficient pagination

backend/tests/bridge_event_listener_test.rs (new)

• 29 integration tests: config validation, topic filtering, parse_event fidelity, extract helpers, cursor format, HTTP endpoint routing, method rejection, Horizon page deserialisation

.github/workflows/backend.yml

• postgres:16 with --health-cmd pg_isready health check
• DATABASE_URL env var set at job level
• cargo test --all-features

pr #942

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