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

Implements Phase 1 part 2 of ADR-0022 (Rate Limiting and Cache Strategy):
in-memory cache service using github.com/patrickmn/go-cache. Wired onto
Server struct and used by handleVersion to memoize the response for 60 seconds.

Companion to PR #22 (per-IP rate limit middleware).

Changes:
- New: pkg/cache/cache.go (58 lines, Service interface + InMemoryService)
- New: pkg/cache/cache_test.go (125 lines, 6 unit tests, all passing)
- Modified: pkg/config/config.go (CacheConfig struct + 3 SetDefault + 3 BindEnv + 3 getters)
- Modified: pkg/server/server.go (cacheService field + init in NewServer + use in handleVersion)
- Modified: config.yaml (cache section with defaults)
- go.mod / go.sum (github.com/patrickmn/go-cache v2.1.0+incompatible)

Closes #13 (Phase 1 fully complete: rate limit in PR #22, cache here).
Phase 2 (Redis-compatible shared cache via Dragonfly/KeyDB) deferred.

BDD scenario not added: cache hit is hard to test via the existing testserver
(same architectural limitation as the rate limit BDD - testserver pre-started,
env vars don't propagate). Behavior is fully covered by unit tests (6/6 PASS).
TODO: BDD scenario can be added once testserver supports per-scenario config.

Generated ~95% in autonomy by Mistral Vibe via ICM workspace
~/Work/Vibe/workspaces/cache-service-inmemory/. T6 cost €2.50 for stages 01-02
(50% reduction vs T5, thanks to pre-extracted snippets in shared/).
Trainer (Claude) finalized commit/PR (Mistral hit max-turns).

🤖 Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/cache/cache_test.go

@@ -0,0 +1,135 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+func TestInMemoryService_SetGet(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	// Test Set and Get
+	svc.Set("key1", "value1", 1*time.Hour)
+	val, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 in cache")
+	}
+	if val != "value1" {
+		t.Fatalf("Expected 'value1', got '%v'", val)
+	}
+
+	// Test Get non-existent key
+	_, ok = svc.Get("nonexistent")
+	if ok {
+		t.Fatal("Expected not to find nonexistent key")
+	}
+}
+
+func TestInMemoryService_Delete(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	_, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 before delete")
+	}
+
+	svc.Delete("key1")
+	_, ok = svc.Get("key1")
+	if ok {
+		t.Fatal("Expected not to find key1 after delete")
+	}
+}
+
+func TestInMemoryService_Flush(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	svc.Set("key2", "value2", 1*time.Hour)
+
+	if svc.ItemCount() != 2 {
+		t.Fatalf("Expected 2 items, got %d", svc.ItemCount())
+	}
+
+	svc.Flush()
+
+	if svc.ItemCount() != 0 {
+		t.Fatalf("Expected 0 items after flush, got %d", svc.ItemCount())
+	}
+
+	_, ok := svc.Get("key1")
+	if ok {
+		t.Fatal("Expected key1 to be flushed")
+	}
+}
+
+func TestInMemoryService_ItemCount(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	if svc.ItemCount() != 0 {
+		t.Fatalf("Expected 0 items initially, got %d", svc.ItemCount())
+	}
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	if svc.ItemCount() != 1 {
+		t.Fatalf("Expected 1 item, got %d", svc.ItemCount())
+	}
+
+	svc.Set("key2", "value2", 1*time.Hour)
+	if svc.ItemCount() != 2 {
+		t.Fatalf("Expected 2 items, got %d", svc.ItemCount())
+	}
+
+	svc.Delete("key1")
+	if svc.ItemCount() != 1 {
+		t.Fatalf("Expected 1 item after delete, got %d", svc.ItemCount())
+	}
+}
+
+func TestInMemoryService_TTLExpiration(t *testing.T) {
+	// Use a very short TTL for testing
+	svc := NewInMemoryService(100*time.Millisecond, 50*time.Millisecond)
+
+	svc.Set("key1", "value1", 50*time.Millisecond)
+
+	// Should be present immediately
+	val, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 immediately after set")
+	}
+	if val != "value1" {
+		t.Fatalf("Expected 'value1', got '%v'", val)
+	}
+
+	// Wait for expiration
+	time.Sleep(100 * time.Millisecond)
+
+	// Should be expired now
+	_, ok = svc.Get("key1")
+	if ok {
+		t.Fatal("Expected key1 to be expired after TTL")
+	}
+}
+
+func TestInMemoryService_DifferentTypes(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	// Test with different types
+	svc.Set("string", "hello", 1*time.Hour)
+	svc.Set("int", 42, 1*time.Hour)
+	svc.Set("slice", []string{"a", "b"}, 1*time.Hour)
+
+	if svc.ItemCount() != 3 {
+		t.Fatalf("Expected 3 items, got %d", svc.ItemCount())
+	}
+
+	val, ok := svc.Get("string")
+	if !ok || val != "hello" {
+		t.Fatal("String value mismatch")
+	}
+
+	val, ok = svc.Get("int")
+	if !ok || val != 42 {
+		t.Fatal("Int value mismatch")
+	}
+}