feat: Implement configuration management and DNS provider integration

- Added configuration management using Viper in internal/config/config.go
- Implemented ClientConfig, ServerConfig, TLSConfig, HetznerConfig, UpstreamConfig, and main Config struct.
- Created LoadConfig function to read and validate configuration files.
- Developed Hetzner DNS provider in internal/provider/hetzner/hetzner.go with methods for updating DNS records.
- Added comprehensive unit tests for configuration loading and Hetzner provider functionality.
- Established HTTP server with metrics and update endpoint in internal/server/server.go.
- Implemented request handling, authorization, and error management in the server.
- Created integration tests for the Hetzner provider API interactions.
- Removed legacy dynamic DNS integration tests in favor of the new API-based approach.

internal/provider/hetzner/hetzner_api_test.go

@@ -0,0 +1,91 @@
+package hetzner_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.cloonar.com/cloonar/updns/internal/provider/hetzner"
+)
+
+func TestUpdateRecordFullAPILifecycle(t *testing.T) {
+	domain := "test.example.com"
+	ip := "1.2.3.4"
+	zoneName := "example.com"
+	zoneID := "zone-123"
+	recID := "rec-456"
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/zones":
+			// Query zones by name
+			resp := map[string]interface{}{
+				"zones": []map[string]string{{"id": zoneID, "name": zoneName}},
+			}
+			json.NewEncoder(w).Encode(resp)
+		case r.Method == http.MethodGet && r.URL.Path == "/records":
+			// Query records by zone_id
+			resp := map[string]interface{}{
+				"records": []map[string]string{{"id": recID, "name": domain, "value": "0.0.0.0", "type": "A"}},
+			}
+			json.NewEncoder(w).Encode(resp)
+		case r.Method == http.MethodPut && r.URL.Path == "/records/"+recID:
+			// Validate update payload
+			body, _ := io.ReadAll(r.Body)
+			var payload map[string]string
+			json.Unmarshal(body, &payload)
+			if payload["value"] != ip {
+				t.Errorf("expected update value %s, got %s", ip, payload["value"])
+			}
+			w.WriteHeader(http.StatusOK)
+		default:
+			http.Error(w, "not found", http.StatusNotFound)
+		}
+	}))
+	defer ts.Close()
+
+	provider := hetzner.NewProviderWithURL("token", ts.URL)
+	if err := provider.UpdateRecord(context.Background(), domain, ip); err != nil {
+		t.Fatalf("full API lifecycle failed: %v", err)
+	}
+}
+
+func TestUpdateRecordZoneNotFound(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode(map[string]interface{}{"zones": []map[string]string{}})
+	}))
+	defer ts.Close()
+
+	provider := hetzner.NewProviderWithURL("token", ts.URL)
+	err := provider.UpdateRecord(context.Background(), "nozone.example", "1.1.1.1")
+	if err == nil || !strings.Contains(err.Error(), "zone example not found") {
+		t.Fatalf("expected zone not found error, got %v", err)
+	}
+}
+
+func TestUpdateRecordRecordNotFound(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/zones":
+			json.NewEncoder(w).Encode(map[string]interface{}{
+				"zones": []map[string]string{{"id": "z", "name": "example.com"}},
+			})
+		case "/records":
+			json.NewEncoder(w).Encode(map[string]interface{}{"records": []map[string]string{}})
+		default:
+			http.Error(w, "not found", http.StatusNotFound)
+		}
+	}))
+	defer ts.Close()
+
+	provider := hetzner.NewProviderWithURL("token", ts.URL)
+	err := provider.UpdateRecord(context.Background(), "missing.example.com", "1.1.1.1")
+	if err == nil || !strings.Contains(err.Error(), "record missing.example.com not found") {
+		t.Fatalf("expected record not found error, got %v", err)
+	}
+}