C-137: Reconciliation test: on-chain vs DB drift heals

**Owner:** @kh0ra  ·  **Reviewer:** @mehmethayirli
**Milestone:** M10 QA, E2E & final tests

Injected drift auto-corrects; alert fires.

**Acceptance criteria**
- Drift auto-corrects.

_Tracked in docs/MAINNET_ROADMAP.md (C-137)._