✨ feat(cache): add in-memory cache service (ADR-0022 Phase 1 part 2) (#23)

Phase 1 part 2 of ADR-0022 (companion to PR #22 rate-limit). In-memory cache service via go-cache, used by /api/version (60s TTL).

6/6 unit tests pass. ~95% Mistral autonomous via ICM workspace, cost €2.50 stages 01-02 (50% reduction vs T5 thanks to pre-extracted snippets in shared/).

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/cache/cache.go

@@ -0,0 +1,56 @@
+package cache
+
+import (
+	"time"
+
+	gocache "github.com/patrickmn/go-cache"
+)
+
+// Service defines the interface for cache operations
+type Service interface {
+	Set(key string, value interface{}, ttl time.Duration)
+	Get(key string) (interface{}, bool)
+	Delete(key string)
+	Flush()
+	ItemCount() int
+}
+
+// InMemoryService implements Service using go-cache library
+type InMemoryService struct {
+	cache *gocache.Cache
+}
+
+// NewInMemoryService creates a new in-memory cache service
+// defaultTTL: default time-to-live for cache items
+// cleanupInterval: interval at which expired items are cleaned up
+func NewInMemoryService(defaultTTL, cleanupInterval time.Duration) Service {
+	c := gocache.New(defaultTTL, cleanupInterval)
+	return &InMemoryService{cache: c}
+}
+
+// Set stores a value in the cache with the specified TTL
+func (s *InMemoryService) Set(key string, value interface{}, ttl time.Duration) {
+	s.cache.Set(key, value, ttl)
+}
+
+// Get retrieves a value from the cache
+// Returns the value and true if found, nil and false if not found or expired
+func (s *InMemoryService) Get(key string) (interface{}, bool) {
+	val, found := s.cache.Get(key)
+	return val, found
+}
+
+// Delete removes an item from the cache
+func (s *InMemoryService) Delete(key string) {
+	s.cache.Delete(key)
+}
+
+// Flush clears all items from the cache
+func (s *InMemoryService) Flush() {
+	s.cache.Flush()
+}
+
+// ItemCount returns the number of items currently in the cache
+func (s *InMemoryService) ItemCount() int {
+	return s.cache.ItemCount()
+}