Add /api/v1/soroban/contract-calls time-series endpoint

[christabel888]

Context: Second Soroban Dashboard endpoint, same data source as #21.

Where:

• src/api/soroban.rs or src/api/contract_events.rs

What to do:

• Time-series of contract-call (event) count per day from contract_events.

Suggested approach:

1. Same file as Add /api/v1/soroban/active-contracts endpoint #21. COUNT(*) from contract_events grouped by day.
2. Consider whether "contract calls" means distinct events or distinct transactions -- one transaction can emit multiple events, so pick a definition and document it.

Watch out for:

• Same Add Soroban contract-invocation ingestion pipeline in src/ingestion #20 dependency as Add /api/v1/soroban/active-contracts endpoint #21.

Definition of done:

• GET /api/v1/soroban/contract-calls returns {date, count} from real contract_events data