Add GeoIP support for unique IP aggregation

- Introduced GeoIP configuration options in config.example.yaml to enable geolocation lookups for the /api/agg/unique-ips endpoint.
- Updated the aggregate handler to include optional GeoIP data in responses, enriching unique IP information with country and city details, as well as ASN data if available.
- Enhanced documentation in AGGREGATE.md and README.md to reflect the new GeoIP functionality and its usage.
- Added a dependency on the geoip2-golang library in go.mod for GeoIP lookups.
- Modified tests to accommodate the new GeoIP integration in the aggregate handler.

internal/geoip/download.go

@@ -0,0 +1,113 @@
+package geoip
+
+import (
+	"compress/gzip"
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// DownloadMMDB downloads an MMDB URL to destPath. If the URL ends with `.gz`, the body is gunzipped.
+// Otherwise the response body is written as-is (raw `.mmdb`, e.g. GitHub raw).
+func DownloadMMDB(ctx context.Context, client *http.Client, url, destPath string) error {
+	if strings.HasSuffix(strings.ToLower(strings.TrimSpace(url)), ".gz") {
+		return downloadGzippedMMDB(ctx, client, url, destPath)
+	}
+	return downloadRawMMDB(ctx, client, url, destPath)
+}
+
+func downloadRawMMDB(ctx context.Context, client *http.Client, url, destPath string) error {
+	if client == nil {
+		client = http.DefaultClient
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return err
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return fmt.Errorf("geoip download: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("geoip download: http %s", resp.Status)
+	}
+	dir := filepath.Dir(destPath)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("geoip mkdir: %w", err)
+	}
+	tmp, err := os.CreateTemp(dir, "geolite-*.mmdb.part")
+	if err != nil {
+		return err
+	}
+	tmpPath := tmp.Name()
+	_, copyErr := io.Copy(tmp, resp.Body)
+	closeErr := tmp.Close()
+	if copyErr != nil {
+		_ = os.Remove(tmpPath)
+		return copyErr
+	}
+	if closeErr != nil {
+		_ = os.Remove(tmpPath)
+		return closeErr
+	}
+	_ = os.Remove(destPath)
+	if err := os.Rename(tmpPath, destPath); err != nil {
+		_ = os.Remove(tmpPath)
+		return fmt.Errorf("geoip rename: %w", err)
+	}
+	return nil
+}
+
+func downloadGzippedMMDB(ctx context.Context, client *http.Client, url, destPath string) error {
+	if client == nil {
+		client = http.DefaultClient
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return err
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return fmt.Errorf("geoip download: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("geoip download: http %s", resp.Status)
+	}
+	gzr, err := gzip.NewReader(resp.Body)
+	if err != nil {
+		return fmt.Errorf("geoip gzip: %w", err)
+	}
+	defer gzr.Close()
+
+	dir := filepath.Dir(destPath)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("geoip mkdir: %w", err)
+	}
+	tmp, err := os.CreateTemp(dir, "geolite-*.mmdb.part")
+	if err != nil {
+		return err
+	}
+	tmpPath := tmp.Name()
+	_, copyErr := io.Copy(tmp, gzr)
+	closeErr := tmp.Close()
+	if copyErr != nil {
+		_ = os.Remove(tmpPath)
+		return copyErr
+	}
+	if closeErr != nil {
+		_ = os.Remove(tmpPath)
+		return closeErr
+	}
+	_ = os.Remove(destPath)
+	if err := os.Rename(tmpPath, destPath); err != nil {
+		_ = os.Remove(tmpPath)
+		return fmt.Errorf("geoip rename: %w", err)
+	}
+	return nil
+}