feat: add Stellar event indexing service

[fathiaoyinloye]

Closes #129

Description

Implement a backend event indexing service that polls Stellar/Soroban contract events and synchronizes them into PostgreSQL, making blockchain activity queryable through the backend API instead of requiring direct contract queries from the frontend.

What changed

• Added a Stellar/Soroban event indexer service.
• Added support for polling registered contracts for new events.
• Processes events in ledger sequence order.
• Added checkpoint tracking to resume indexing after restarts.
• Added deduplication using:
  • contract_id
  • ledger
  • event_index
• Added support for indexing:
  • CredentialIssued
  • CredentialRevoked
  • CourseCreated
  • EnrollmentCreated
  • AchievementMinted
  • PaymentReceived
  • ProfileUpdated
• Added the indexed_events database model and migration with JSONB event payload storage.
• Added domain table synchronization for relevant event types.
• Added configurable polling with a default interval of 5 seconds.
• Added indexer health information including:
  • Running status
  • Last indexed ledger
  • Events processed
  • Current ledger lag
• Added graceful shutdown handling to finish the current batch and persist the checkpoint.
• Added error handling for malformed events so individual failures do not stop the indexer.
• Added admin controls to start and stop the indexer.
• Verified contract event emission consistency in event_logger.rs.
• Documented the event indexer environment variables in SETUP_GUIDE.md.

Verification

• Events are indexed into PostgreSQL after contract execution.
• Indexing resumes from the last processed ledger after restart.
• Reprocessing the same ledger range does not create duplicate records.
• Malformed events are logged and skipped without stopping the indexer.
• Health endpoint exposes event indexer status and ledger lag.
• Indexer polling interval is configurable.
• Graceful shutdown saves the latest checkpoint.
• Database connections are reused safely without exhausting the connection pool.
• Tests cover event processing, checkpoint recovery, deduplication, malformed events, and indexer lifecycle behavior.

Configuration

The following environment variables are supported:

EVENT_INDEXER_ENABLED=true
EVENT_INDEXER_POLL_INTERVAL=5000