[#164] Blood Request Entity and Workflow

[Goodnessukaigwe]

📌 Summary

Implements the core Blood Request entity, item relationships, and lifecycle workflow (#164).

🚀 What was done

• Created the BloodRequest entity extending BaseEntity.
• Defined RequestStatus and UrgencyLevel enumerations for workflow management.
• Implemented RequestItem for supporting multi-blood-type requests within a single order.
• Established relationships between BloodRequest, Hospital, and RequestStatusHistory.
• Added blockchain reference fields for decentralized verification and SLA tracking fields for performance monitoring.
• Optimized database performance with indexes on status and hospital identifiers.

🧠 Key Logic

• Established a One-to-Many relationship between BloodRequest and RequestItem to allow bulk requests.
• Implemented a RequestStatusHistory audit trail to record every state transition.
• Integrated UrgencyLevel logic to facilitate SLA (Service Level Agreement) calculations (e.g., Critical = 1hr response).
• Included blockchain transaction hash fields to link on-chain Stellar operations with the local database.

🔒 Validations

• Strict Enum validation for RequestStatus (PENDING, MATCHED, APPROVED, FULFILLED, CANCELLED, REJECTED).
• Validation for UrgencyLevel to ensure prioritized handling of CRITICAL/URGENT requests.
• Constraints ensuring a BloodRequest must be associated with a valid Hospital and at least one RequestItem.

💰 Side Effects

• Database schema migration to include BloodRequest, RequestItem, and RequestStatusHistory tables.
• Performance overhead for indexing status-based queries.
• Creation of audit records upon every status modification.

🧪 Tests

• ✅ Entity instantiation and property assignment.
• ✅ Successful mapping of One-to-Many RequestItem relationship.
• ✅ Validation of RequestStatus transitions.
• ✅ Correct association with the Hospital entity.
• ✅ SLA field persistence and blockchain reference storage.

📎 Notes

• Designed to be compatible with the existing BaseEntity architecture.
• Future-proofed with blockchain fields to align with the project's Stellar integration goals.

Closes #164