Implement a Thread-Safe Pub-Sub Service

Implement a simple server-side publish-subscribe system using Python's standard threading library.

Requirements:

• Multiple publisher threads may publish messages concurrently.
• Multiple subscriber threads may subscribe to topics concurrently.
• A subscriber can subscribe to and unsubscribe from a topic.
• When a message is published to a topic, all currently subscribed subscribers should receive the message.
• The implementation must avoid race conditions and deadlocks.
• Use Python threading primitives such as Lock, RLock, Condition, Event, and thread-safe queues where appropriate.
• Discuss delivery semantics, shutdown behavior, and how to avoid one slow subscriber blocking all publishers.