Observability hardening: prod logging levels, structured logs, secure Actuator

[sugan0tech]

• Description: Add Actuator and Micrometer, secure endpoints, and standardize logs.
• Current state: Dev properties expose all Actuator endpoints but no actuator dependency exists. Logging levels are DEBUG/TRACE in dev. No JSON/structured logging.
• Tasks:
  • Add spring-boot-starter-actuator and micrometer-registry-prometheus.
  • Configure prod to expose only health, info, prometheus; require auth/role for any sensitive endpoints.
  • Set prod logging level to INFO; configure structured JSON logs (Logback encoder or layout).
  • Add /actuator/health readiness/liveness groups for Docker healthcheck.
• Acceptance criteria:
  • Actuator available and secured; logs are structured in prod.
• References: application-dev.properties, pom.xml.