fix(indexer): make event persistence atomic (#168)

[edehvictor]

Closes #168

Changes

• wrap the processed-event lookup, every domain handler write, and the final idempotency marker in one Prisma interactive transaction
• thread the transaction-scoped Prisma client through all event handlers and shared user upserts
• add a regression test that injects a mid-handler failure, verifies the staged investment is rolled back, and proves a later retry commits exactly once

Testing

• npm test -- --runInBand — 10 suites, 118 tests passed
• npm run build — passed
• npx eslint "src/indexer/parsers/event-parser.service.ts" "src/indexer/parsers/event-parser.service.spec.ts" — passed
• git diff --check — passed

Notes

• Repository-wide ESLint on this Windows checkout reports pre-existing CRLF formatting errors in unchanged files; the two files changed by this PR pass the configured lint rules.