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/server/server_test.go

@@ -0,0 +1,31 @@
+package server_test
+
+import (
+	"strings"
+	"testing"
+
+	"git.cloonar.com/cloonar/updns/internal/config"
+	"git.cloonar.com/cloonar/updns/internal/server"
+)
+
+func TestStartServerUnsupportedProvider(t *testing.T) {
+	cfg := &config.Config{
+		Server: config.ServerConfig{
+			BindAddress: "127.0.0.1:0",
+			TLS: config.TLSConfig{
+				Enabled: false,
+			},
+		},
+		Upstream: config.UpstreamConfig{
+			Provider: "unknown",
+		},
+		Clients: map[string]config.ClientConfig{},
+	}
+	err := server.StartServer(cfg)
+	if err == nil {
+		t.Fatal("expected error for unsupported provider, got nil")
+	}
+	if !strings.Contains(err.Error(), "unsupported provider") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}