Add structured logging with correlation IDs and centralized log aggregation

[Smartdevs17]

Description

Application logs are unstructured and lack correlation IDs, making debugging across services difficult. Implement structured logging with trace context propagation.

Acceptance Criteria

• Structured JSON logging with consistent fields
• Correlation ID propagation across async boundaries
• Log level configuration per module
• Centralized log aggregation (Loki or similar)
• Log query dashboard with filtering
• PII redaction in log output

Technical Scope

• backend/src/middleware/logger.ts
• backend/src/middleware/requestId.js
• Edge: log volume management, sampling strategies, async context propagation