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.go

@@ -0,0 +1,132 @@
+package hetzner
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+
+	pvd "git.cloonar.com/cloonar/updns/internal/provider"
+)
+
+const defaultAPIBase = "https://dns.hetzner.com"
+
+type provider struct {
+	token      string
+	client     *http.Client
+	apiBaseURL string
+}
+
+type zone struct {
+	ID   string `json:"id"`
+	Name string `json:"name"`
+}
+
+type zonesResponse struct {
+	Zones []zone `json:"zones"`
+}
+
+type record struct {
+	ID    string `json:"id"`
+	Name  string `json:"name"`
+	Value string `json:"value"`
+	Type  string `json:"type"`
+}
+
+type recordsResponse struct {
+	Records []record `json:"records"`
+}
+
+// NewProvider creates a Hetzner DNS provider using the official API.
+func NewProvider(token string) pvd.Provider {
+	return &provider{token: token, client: http.DefaultClient, apiBaseURL: defaultAPIBase}
+}
+
+// NewProviderWithURL creates a Hetzner provider with a custom API base URL (for testing).
+func NewProviderWithURL(token, apiBaseURL string) pvd.Provider {
+	return &provider{token: token, client: http.DefaultClient, apiBaseURL: apiBaseURL}
+}
+
+// UpdateRecord updates the DNS record for the given domain to the provided IP.
+func (p *provider) UpdateRecord(ctx context.Context, domain, ip string) error {
+	// Determine zone name (last two labels, or single label for two-part domains)
+	parts := strings.Split(domain, ".")
+	if len(parts) < 2 {
+		return fmt.Errorf("invalid domain: %s", domain)
+	}
+	var zoneName string
+	if len(parts) == 2 {
+		zoneName = parts[1]
+	} else {
+		zoneName = strings.Join(parts[len(parts)-2:], ".")
+	}
+	// Fetch zone ID
+	zonesURL := fmt.Sprintf("%s/zones?name=%s", p.apiBaseURL, zoneName)
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, zonesURL, nil)
+	req.Header.Set("Authorization", "Bearer "+p.token)
+	resp, err := p.client.Do(req)
+	if err != nil {
+		return fmt.Errorf("fetch zones: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("hetzner update failed with status: %s", resp.Status)
+	}
+	var zr zonesResponse
+	if err := json.NewDecoder(resp.Body).Decode(&zr); err != nil {
+		return fmt.Errorf("parsing zones response: %w", err)
+	}
+	if len(zr.Zones) == 0 {
+		return fmt.Errorf("zone %s not found", zoneName)
+	}
+	zoneID := zr.Zones[0].ID
+
+	// Fetch records in zone
+	recsURL := fmt.Sprintf("%s/records?zone_id=%s", p.apiBaseURL, zoneID)
+	req, _ = http.NewRequestWithContext(ctx, http.MethodGet, recsURL, nil)
+	req.Header.Set("Authorization", "Bearer "+p.token)
+	resp, err = p.client.Do(req)
+	if err != nil {
+		return fmt.Errorf("fetch records: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("hetzner update failed with status: %s", resp.Status)
+	}
+	var rr recordsResponse
+	if err := json.NewDecoder(resp.Body).Decode(&rr); err != nil {
+		return fmt.Errorf("parsing records response: %w", err)
+	}
+	var recID string
+	for _, rec := range rr.Records {
+		if rec.Name == domain {
+			recID = rec.ID
+			break
+		}
+	}
+	if recID == "" {
+		return fmt.Errorf("record %s not found", domain)
+	}
+
+	// Update record value
+	updateURL := fmt.Sprintf("%s/records/%s", p.apiBaseURL, recID)
+	body := map[string]string{"value": ip}
+	buf := &bytes.Buffer{}
+	if err := json.NewEncoder(buf).Encode(body); err != nil {
+		return fmt.Errorf("encode update body: %w", err)
+	}
+	req, _ = http.NewRequestWithContext(ctx, http.MethodPut, updateURL, buf)
+	req.Header.Set("Authorization", "Bearer "+p.token)
+	req.Header.Set("Content-Type", "application/json")
+	resp, err = p.client.Do(req)
+	if err != nil {
+		return fmt.Errorf("update record: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("update API status: %s", resp.Status)
+	}
+	return nil
+}