docs: add 'When to Activate' sections to 14 skill definitions

Add activation triggers to skills that were missing them, helping
Claude Code determine when to load each skill contextually.

skills/backend-patterns/SKILL.md

@@ -7,6 +7,16 @@ description: Backend architecture patterns, API design, database optimization, a
 
 Backend architecture patterns and best practices for scalable server-side applications.
 
+## When to Activate
+
+- Designing REST or GraphQL API endpoints
+- Implementing repository, service, or controller layers
+- Optimizing database queries (N+1, indexing, connection pooling)
+- Adding caching (Redis, in-memory, HTTP cache headers)
+- Setting up background jobs or async processing
+- Structuring error handling and validation for APIs
+- Building middleware (auth, logging, rate limiting)
+
 ## API Design Patterns
 
 ### RESTful API Structure