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/config/config_test.go

@@ -0,0 +1,64 @@
+package config_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"git.cloonar.com/cloonar/updns/internal/config"
+)
+
+func TestLoadConfig_Success(t *testing.T) {
+	content := `
+server:
+  bind_address: ":9090"
+  tls:
+    enabled: true
+    cert_file: "cert.pem"
+    key_file: "key.pem"
+upstream:
+  provider: hetzner
+  hetzner:
+    api_token: "token123"
+clients:
+  clientA:
+    secret: "sec"
+    exact:
+      - "foo.com"
+    wildcard:
+      - "bar.com"
+`
+	tmp := filepath.Join(os.TempDir(), "config_test.yaml")
+	if err := os.WriteFile(tmp, []byte(content), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+	defer os.Remove(tmp)
+
+	cfg, err := config.LoadConfig(tmp)
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+	if cfg.Server.BindAddress != ":9090" {
+		t.Errorf("expected bind_address :9090, got %s", cfg.Server.BindAddress)
+	}
+	if !cfg.Server.TLS.Enabled {
+		t.Error("expected TLS enabled")
+	}
+}
+
+func TestLoadConfig_Failure(t *testing.T) {
+	content := `
+clients:
+  clientB:
+    secret: "sec"
+`
+	tmp := filepath.Join(os.TempDir(), "config_fail.yaml")
+	if err := os.WriteFile(tmp, []byte(content), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+	defer os.Remove(tmp)
+
+	if _, err := config.LoadConfig(tmp); err == nil {
+		t.Fatal("expected error for missing fields, got nil")
+	}
+}