feat(auth): implement master key authentication system with child key issuance

Add admin and master authentication layers with JWT support. Replace direct
key creation with hierarchical master/child key system. Update database
schema to support master accounts with configurable limits and epoch-based
key revocation. Add health check endpoint with system status monitoring.

BREAKING CHANGE: Removed direct POST /keys endpoint in favor of master-based
key issuance through /v1/tokens. Database migration requires dropping old User
table and creating Master table with new relationships.

internal/config/config.go

@@ -11,12 +11,17 @@ type Config struct {
 	Postgres PostgresConfig
 	Redis    RedisConfig
 	Log      LogConfig
+	Auth     AuthConfig
 }
 
 type ServerConfig struct {
 	Port string
 }
 
+type AuthConfig struct {
+	JWTSecret string
+}
+
 type PostgresConfig struct {
 	DSN string
 }
@@ -51,6 +56,9 @@ func Load() (*Config, error) {
 			FlushInterval: getEnvDuration("EZ_LOG_FLUSH_MS", 1000),
 			QueueCapacity: getEnvInt("EZ_LOG_QUEUE", 10000),
 		},
+		Auth: AuthConfig{
+			JWTSecret: getEnv("EZ_JWT_SECRET", "change_me_in_production"),
+		},
 	}, nil
 }