[PERFORMANCE] [P0] Add caching implementation across service layer

[BodieCoding]

Problem Description

No caching is implemented, causing every request to hit the database and poor performance.

Current State

• Every request hits database
• No caching strategy
• Poor response times (estimated 500ms-2s)
• Database overload potential

Desired State

• Redis caching implemented
• 80%+ cache hit rate
• <200ms API response times
• 60% reduction in database calls

Acceptance Criteria

• Redis caching infrastructure setup
• Template and variable caching
• Cache invalidation strategy
• Cache hit rate monitoring
• Performance improvement measured
• Cache consistency ensured

Implementation Approach

1. Setup Redis infrastructure
2. Implement caching decorators
3. Add cache invalidation logic
4. Implement cache warming
5. Add monitoring and metrics
6. Performance testing

Files Affected

• PromptStudio.Core/Services/ (all services)
• Infrastructure configuration
• Dependency injection setup

Related Analysis

Source: PROMPTSTUDIO_CORE_ANALYSIS_README.md
Section: Performance Analysis, No Caching Implementation

Definition of Done

• Caching infrastructure operational
• Performance targets met (<200ms)
• Monitoring dashboards active
• Load testing passed
• Documentation updated

Related to #1