Map contract events to DB rows in the indexer #7

Context

src/indexer.ts only console.logs events. The DB is never updated, so API responses are stale.

Acceptance criteria

• On request_created: upsert a PaymentRequest row (status='Open').
• On request_paid: set that row's status='Paid' and store payer.
• On request_cancelled: set status='Cancelled'.
• Idempotent: re-processing the same ledger window does not duplicate rows.

Technical notes

• Parse event topics/payload per the contract's emitted shape (see contracts issue on structured events).
• Key rows on (contract, requestId).
• Update Cursor after each successful window.

Testing

• Unit-test the mapping function with a fixture event object asserting the resulting DB write.