refactor: rewrite handler-base as Go module

Replace Python handler-base library with Go module providing:
- config: environment-based configuration
- health: HTTP health/readiness server for k8s probes
- natsutil: NATS/JetStream client with msgpack serialization
- telemetry: OpenTelemetry tracing and metrics setup
- clients: HTTP clients for LLM, embeddings, reranker, STT, TTS
- handler: base Handler runner wiring NATS + health + telemetry

Implements ADR-0061 Phase 1.

config/config.go

@@ -0,0 +1,145 @@
+// Package config provides environment-based configuration for handler services.
+package config
+
+import (
+	"os"
+	"strconv"
+	"time"
+)
+
+// Settings holds base configuration for all handler services.
+// Values are loaded from environment variables with sensible defaults.
+type Settings struct {
+	// Service identification
+	ServiceName      string
+	ServiceVersion   string
+	ServiceNamespace string
+	DeploymentEnv    string
+
+	// NATS configuration
+	NATSURL        string
+	NATSUser       string
+	NATSPassword   string
+	NATSQueueGroup string
+
+	// Redis/Valkey configuration
+	RedisURL      string
+	RedisPassword string
+
+	// Milvus configuration
+	MilvusHost       string
+	MilvusPort       int
+	MilvusCollection string
+
+	// Service endpoints
+	EmbeddingsURL string
+	RerankerURL   string
+	LLMURL        string
+	TTSURL        string
+	STTURL        string
+
+	// OpenTelemetry configuration
+	OTELEnabled  bool
+	OTELEndpoint string
+	OTELUseHTTP  bool
+
+	// HyperDX configuration
+	HyperDXEnabled  bool
+	HyperDXAPIKey   string
+	HyperDXEndpoint string
+
+	// MLflow configuration
+	MLflowTrackingURI    string
+	MLflowExperimentName string
+	MLflowEnabled        bool
+
+	// Health check configuration
+	HealthPort int
+	HealthPath string
+	ReadyPath  string
+
+	// Timeouts
+	HTTPTimeout time.Duration
+	NATSTimeout time.Duration
+}
+
+// Load creates a Settings populated from environment variables with defaults.
+func Load() *Settings {
+	return &Settings{
+		ServiceName:      getEnv("SERVICE_NAME", "handler"),
+		ServiceVersion:   getEnv("SERVICE_VERSION", "1.0.0"),
+		ServiceNamespace: getEnv("SERVICE_NAMESPACE", "ai-ml"),
+		DeploymentEnv:    getEnv("DEPLOYMENT_ENV", "production"),
+
+		NATSURL:        getEnv("NATS_URL", "nats://nats.ai-ml.svc.cluster.local:4222"),
+		NATSUser:       getEnv("NATS_USER", ""),
+		NATSPassword:   getEnv("NATS_PASSWORD", ""),
+		NATSQueueGroup: getEnv("NATS_QUEUE_GROUP", ""),
+
+		RedisURL:      getEnv("REDIS_URL", "redis://valkey.ai-ml.svc.cluster.local:6379"),
+		RedisPassword: getEnv("REDIS_PASSWORD", ""),
+
+		MilvusHost:       getEnv("MILVUS_HOST", "milvus.ai-ml.svc.cluster.local"),
+		MilvusPort:       getEnvInt("MILVUS_PORT", 19530),
+		MilvusCollection: getEnv("MILVUS_COLLECTION", "documents"),
+
+		EmbeddingsURL: getEnv("EMBEDDINGS_URL", "http://embeddings-predictor.ai-ml.svc.cluster.local"),
+		RerankerURL:   getEnv("RERANKER_URL", "http://reranker-predictor.ai-ml.svc.cluster.local"),
+		LLMURL:        getEnv("LLM_URL", "http://vllm-predictor.ai-ml.svc.cluster.local"),
+		TTSURL:        getEnv("TTS_URL", "http://tts-predictor.ai-ml.svc.cluster.local"),
+		STTURL:        getEnv("STT_URL", "http://whisper-predictor.ai-ml.svc.cluster.local"),
+
+		OTELEnabled:  getEnvBool("OTEL_ENABLED", true),
+		OTELEndpoint: getEnv("OTEL_ENDPOINT", "http://opentelemetry-collector.observability.svc.cluster.local:4317"),
+		OTELUseHTTP:  getEnvBool("OTEL_USE_HTTP", false),
+
+		HyperDXEnabled:  getEnvBool("HYPERDX_ENABLED", false),
+		HyperDXAPIKey:   getEnv("HYPERDX_API_KEY", ""),
+		HyperDXEndpoint: getEnv("HYPERDX_ENDPOINT", "https://in-otel.hyperdx.io"),
+
+		MLflowTrackingURI:    getEnv("MLFLOW_TRACKING_URI", "http://mlflow.mlflow.svc.cluster.local:80"),
+		MLflowExperimentName: getEnv("MLFLOW_EXPERIMENT_NAME", ""),
+		MLflowEnabled:        getEnvBool("MLFLOW_ENABLED", true),
+
+		HealthPort: getEnvInt("HEALTH_PORT", 8080),
+		HealthPath: getEnv("HEALTH_PATH", "/health"),
+		ReadyPath:  getEnv("READY_PATH", "/ready"),
+
+		HTTPTimeout: getEnvDuration("HTTP_TIMEOUT", 60*time.Second),
+		NATSTimeout: getEnvDuration("NATS_TIMEOUT", 30*time.Second),
+	}
+}
+
+func getEnv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func getEnvInt(key string, fallback int) int {
+	if v := os.Getenv(key); v != "" {
+		if i, err := strconv.Atoi(v); err == nil {
+			return i
+		}
+	}
+	return fallback
+}
+
+func getEnvBool(key string, fallback bool) bool {
+	if v := os.Getenv(key); v != "" {
+		if b, err := strconv.ParseBool(v); err == nil {
+			return b
+		}
+	}
+	return fallback
+}
+
+func getEnvDuration(key string, fallback time.Duration) time.Duration {
+	if v := os.Getenv(key); v != "" {
+		if f, err := strconv.ParseFloat(v, 64); err == nil {
+			return time.Duration(f * float64(time.Second))
+		}
+	}
+	return fallback
+}

config/config_test.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"os"
+	"testing"
+	"time"
+)
+
+func TestLoadDefaults(t *testing.T) {
+	s := Load()
+	if s.ServiceName != "handler" {
+		t.Errorf("expected default ServiceName 'handler', got %q", s.ServiceName)
+	}
+	if s.HealthPort != 8080 {
+		t.Errorf("expected default HealthPort 8080, got %d", s.HealthPort)
+	}
+	if s.HTTPTimeout != 60*time.Second {
+		t.Errorf("expected default HTTPTimeout 60s, got %v", s.HTTPTimeout)
+	}
+}
+
+func TestLoadFromEnv(t *testing.T) {
+	os.Setenv("SERVICE_NAME", "test-svc")
+	os.Setenv("HEALTH_PORT", "9090")
+	os.Setenv("OTEL_ENABLED", "false")
+	defer func() {
+		os.Unsetenv("SERVICE_NAME")
+		os.Unsetenv("HEALTH_PORT")
+		os.Unsetenv("OTEL_ENABLED")
+	}()
+
+	s := Load()
+	if s.ServiceName != "test-svc" {
+		t.Errorf("expected ServiceName 'test-svc', got %q", s.ServiceName)
+	}
+	if s.HealthPort != 9090 {
+		t.Errorf("expected HealthPort 9090, got %d", s.HealthPort)
+	}
+	if s.OTELEnabled {
+		t.Error("expected OTELEnabled false")
+	}
+}