✨ 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/config/config.go

@@ -28,6 +28,7 @@ type Config struct {
 	Auth      AuthConfig      `mapstructure:"auth"`
 	Database  DatabaseConfig  `mapstructure:"database"`
 	RateLimit RateLimitConfig `mapstructure:"rate_limit"`
+	Cache     CacheConfig     `mapstructure:"cache"`
 }
 
 // ServerConfig holds server-related configuration
@@ -105,6 +106,13 @@ type RateLimitConfig struct {
 	BurstSize         int  `mapstructure:"burst_size"`
 }
 
+// CacheConfig holds cache configuration
+type CacheConfig struct {
+	Enabled                bool `mapstructure:"enabled"`
+	DefaultTTLSeconds      int  `mapstructure:"default_ttl_seconds"`
+	CleanupIntervalSeconds int  `mapstructure:"cleanup_interval_seconds"`
+}
+
 // VersionInfo holds application version information
 type VersionInfo struct {
 	Version   string `mapstructure:"-"` // Set via ldflags
@@ -202,6 +210,11 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("rate_limit.requests_per_minute", 60)
 	v.SetDefault("rate_limit.burst_size", 10)
 
+	// Cache defaults
+	v.SetDefault("cache.enabled", true)
+	v.SetDefault("cache.default_ttl_seconds", 300)
+	v.SetDefault("cache.cleanup_interval_seconds", 600)
+
 	// Auth defaults
 	v.SetDefault("auth.jwt_secret", "default-secret-key-please-change-in-production")
 	v.SetDefault("auth.admin_master_password", "admin123")
@@ -266,6 +279,11 @@ func LoadConfig() (*Config, error) {
 	v.BindEnv("rate_limit.requests_per_minute", "DLC_RATE_LIMIT_REQUESTS_PER_MINUTE")
 	v.BindEnv("rate_limit.burst_size", "DLC_RATE_LIMIT_BURST_SIZE")
 
+	// Cache environment variables
+	v.BindEnv("cache.enabled", "DLC_CACHE_ENABLED")
+	v.BindEnv("cache.default_ttl_seconds", "DLC_CACHE_DEFAULT_TTL_SECONDS")
+	v.BindEnv("cache.cleanup_interval_seconds", "DLC_CACHE_CLEANUP_INTERVAL_SECONDS")
+
 	// Database environment variables
 	v.BindEnv("database.host", "DLC_DATABASE_HOST")
 	v.BindEnv("database.port", "DLC_DATABASE_PORT")
@@ -428,6 +446,27 @@ func (c *Config) GetRateLimitBurstSize() int {
 	return c.RateLimit.BurstSize
 }
 
+// GetCacheEnabled returns whether cache is enabled
+func (c *Config) GetCacheEnabled() bool {
+	return c.Cache.Enabled
+}
+
+// GetCacheDefaultTTLSeconds returns the default TTL in seconds for cache items
+func (c *Config) GetCacheDefaultTTLSeconds() int {
+	if c.Cache.DefaultTTLSeconds <= 0 {
+		return 300
+	}
+	return c.Cache.DefaultTTLSeconds
+}
+
+// GetCacheCleanupIntervalSeconds returns the cleanup interval in seconds for cache
+func (c *Config) GetCacheCleanupIntervalSeconds() int {
+	if c.Cache.CleanupIntervalSeconds <= 0 {
+		return 600
+	}
+	return c.Cache.CleanupIntervalSeconds
+}
+
 // GetDatabaseHost returns the database host
 func (c *Config) GetDatabaseHost() string {
 	if c.Database.Host == "" {