🟡 HIGH: Add custom metrics for business operations

[GitAddRemote]

Problem

No application-level metrics beyond default Actuator

Solution

Add custom metrics using Micrometer:

• @timed for critical operations
• Counters for business events
• Auth metrics (registrations, logins, rate limits)
• JWT metrics (generated, revoked, validated)
• Kafka metrics (sent, failed, consumed)

Acceptance Criteria

• @timed annotations on critical methods
• Custom counters for business events
• Metrics exposed via /actuator/prometheus
• Grafana dashboard created

Priority

🟡 High - Essential for production monitoring