Implement Contract Event System


**Description:** 
Emit structured events for contract operations.

**Tasks:** 
- \[ \] Create DonationReceived event 
- \[ \] Create WithdrawalProcessed event 
- \[ \] Structure event payload 
- \[ \] Emit events in contract functions 
- \[ \] Test event emission

**Acceptance Criteria:** 
- Events emitted correctly  
- Events readable by indexers 
- Event data structured
