[Feature] Implement Low-Stock Alert Thresholds #8

Problem

Inventory service has no configurable thresholds for low-stock alerts. Alert logic not implemented.

Impact

• Hospitals cannot proactively reorder
• Critical shortages go unnoticed
• Emergency requests fail due to stock depletion

Solution

• Add hospital-specific low-stock thresholds
• Publish alerts when stock drops below threshold
• Track alert history

Acceptance Criteria

• Threshold configuration stored per hospital
• Alerts published to RabbitMQ on threshold breach
• Low-stock alerts sent via notification service
• Threshold update API endpoint