[Docs] Document Database Schema #17

Missing Information

• Collection schemas
• Index information
• Relationships between collections
• TTL policies

Solution

Create schema documentation:

• Emergency Request schema
• Donor schema
• Inventory schema
• EventLog schema
• SagaStep schema
• NotificationLog schema

Files to Create

• DATABASE_SCHEMA.md (root directory)
• Include mermaid diagrams or JSON schema

Acceptance Criteria

• All collections documented
• Fields described with types
• Indexes listed
• Relationships shown
• Mermaid ER diagram included