Add Observability & Health Checks

[viceda-s]

Problem: Zero visibility into application health or metrics for orchestration tools like Kubernetes.

Acceptance Criteria:

• Add spring-boot-starter-actuator to pom.xml.
• Expose /actuator/health and /actuator/info endpoints.
• Add structured SLF4J business logging inside WorkoutService and AuthService for tracing key events (e.g., "Workout created", "User registered").

[viceda-s]

Added spring-boot-starter-actuator, exposed /actuator/health and /actuator/info, and added structured business logging via SLF4J in service layers while avoiding PII exposure.