feat(api): implement ListEvents gRPC handler

[Depo-dev]

Summary

EventsServiceImpl::list_events currently returns Status::unimplemented. Needs a real Postgres query with filters and cursor pagination.

Acceptance Criteria

• Accept ListEventsRequest filters: contract_id, topic_0, topic_1, ledger_from, ledger_to
• Build a dynamic WHERE clause from whichever filters are present (all optional)
• Cursor-based pagination: accept cursor (last seen UUID), return up to 200 rows, include next_cursor in response
• Map soroban_events rows → proto Event messages
• Return Status::internal with a safe message on DB errors (no raw SQL in error responses)
• Integration test: seed 5 events, query by contract_id, assert correct count and field values

Files

crates/api/src/services/events.rs

Notes

The API crate needs a sqlx::PgPool injected into EventsServiceImpl. Add it to the struct and wire it in main.rs.