fix(api): also add etag

_oas/openapi.yaml

@@ -49,7 +49,12 @@ paths:
               description: "The Cache-Control header provides important info on how long a response may be considered fresh"
               schema:
                 type: string
-              example: "Cache-Control: max-age=3600, public, must-revalidate"
+              example: "max-age=3600, public, must-revalidate"
+            ETag:
+              description: "The ETag header provides a unique identifier for the response"
+              schema:
+                type: string
+              example: "W/\"123456789\""
           content:
             "image/svg+xml":
               schema:

internal/api/badge.go

@@ -3,6 +3,8 @@ package api
 import (
 	"bytes"
 	"context"
+	"fmt"
+	"hash/crc32"
 	"io"
 	"net/http"
 	"net/url"
@@ -27,6 +29,11 @@ func generateBadgePath(name, text, style string) string {
 	}, "-")
 }
 
+func etag(name string, data []byte) string {
+	crc := crc32.ChecksumIEEE(data)
+	return fmt.Sprintf(`W/"%s-%d-%08X"`, name, len(data), crc)
+}
+
 func (s Server) GetTelegramBadge(ctx context.Context, params oas.GetTelegramBadgeParams) (*oas.GetTelegramBadgeOKHeaders, error) {
 	members := map[string]int{
 		"gotd_en":   237,
@@ -60,6 +67,7 @@ func (s Server) GetTelegramBadge(ctx context.Context, params oas.GetTelegramBadg
 
 	return &oas.GetTelegramBadgeOKHeaders{
 		CacheControl: oas.NewOptString("no-cache"),
+		ETag:         oas.NewOptString(etag(params.GroupName, data)),
 		Response: oas.GetTelegramBadgeOK{
 			Data: bytes.NewReader(data),
 		},