Implement a Design Pattern

Implement the following design pattern in the specified language:

Design pattern: [e.g. Observer / Strategy / Factory / Singleton / Decorator / Repository / Command / Builder]
Language: [e.g. TypeScript / Python / Go / Java / C#]
Use case: [a real, specific scenario where this pattern solves a problem — not just a toy example]

Provide:
1. Brief explanation: what problem this pattern solves and when to use it (and when NOT to)
2. UML diagram description: describe the class/interface structure in plain text
3. Full implementation: clean, working code with the pattern applied to the use case
4. Usage example: show how client code uses the pattern
5. Benefits in this context: what specifically does this pattern give you here?
6. Trade-offs: what complexity or overhead does it add?
7. Common mistake: the most frequent way developers misapply this pattern

Write production-quality code with appropriate typing, error handling, and naming.