feat: reword phase to epic, update mkdocs

docs/content/stories/epic5/5.1-cache-system.md

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+# Story 5.1: Cache System (Redis)
+
+## Metadata
+- **Story ID**: 5.1
+- **Title**: Cache System (Redis)
+- **Epic**: 5 - Infrastructure Adapters
+- **Status**: Pending
+- **Priority**: High
+- **Estimated Time**: 4-5 hours
+- **Dependencies**: 1.1
+
+## Goal
+Implement a complete Redis-based caching system with a clean interface that can be swapped for other cache implementations.
+
+## Description
+This story implements a Redis cache adapter with a clean interface, allowing modules to cache data efficiently. The cache system supports TTL, key-based operations, and optional cache middleware for HTTP responses.
+
+## Deliverables
+
+### 1. Cache Interface (`pkg/infra/cache/cache.go`)
+- `Cache` interface with:
+  - `Get(ctx context.Context, key string) ([]byte, error)`
+  - `Set(ctx context.Context, key string, value []byte, ttl time.Duration) error`
+  - `Delete(ctx context.Context, key string) error`
+  - `Exists(ctx context.Context, key string) (bool, error)`
+  - `Clear(ctx context.Context) error`
+
+### 2. Redis Implementation (`internal/infra/cache/redis_cache.go`)
+- Redis client setup
+- Connection pooling
+- All interface methods implemented
+- Error handling
+- Connection health checks
+
+### 3. Configuration
+- Redis config in `config/default.yaml`:
+  - Connection URL
+  - Pool settings
+  - Default TTL
+
+### 4. DI Integration
+- Provider function for Cache
+- Register in DI container
+
+### 5. Optional Cache Middleware
+- HTTP response caching middleware
+- Configurable cache keys
+- TTL per route
+
+## Acceptance Criteria
+- [ ] Cache interface is defined
+- [ ] Redis implementation works correctly
+- [ ] Cache operations (get, set, delete) work
+- [ ] TTL is respected
+- [ ] Cache is injectable via DI
+- [ ] Configuration is loaded from config
+- [ ] Optional middleware works
+
+## Files to Create/Modify
+- `pkg/infra/cache/cache.go` - Cache interface
+- `internal/infra/cache/redis_cache.go` - Redis implementation
+- `internal/di/providers.go` - Add cache provider
+- `config/default.yaml` - Add Redis config
+