[TECHNICAL-DEBT] [P1] Implement repository pattern to reduce DB coupling

[BodieCoding]

Problem Description

Direct database context coupling throughout services makes testing difficult and hurts performance.

Current State

• Services directly use DbContext
• Tight coupling to EF Core
• Difficult unit testing
• No query optimization

Desired State

• Repository pattern implementation
• Loose coupling to data layer
• Easy unit testing with mocks
• Query optimization opportunities

Acceptance Criteria

• Create repository interfaces
• Implement repository classes
• Update services to use repositories
• Add unit tests with mocked repositories
• Maintain all existing functionality
• Add query optimization methods

Implementation Approach

1. Design repository interfaces
2. Implement concrete repositories
3. Add caching to repositories
4. Update dependency injection
5. Update services to use repositories
6. Add comprehensive unit tests

Files Affected

• PromptStudio.Core/Services/PromptService.cs
• New repository interfaces and implementations
• Dependency injection configuration

Related Analysis

Source: PROMPTSTUDIO_CORE_ANALYSIS_README.md
Section: Service Layer Analysis, Direct Database Coupling

Definition of Done

• Repository pattern implemented
• Services updated
• Tests using mocked repositories
• Performance maintained or improved
• Query optimization added

Related to #1, #2, and #5