Update build configuration and improve token handling

- Change CI container image to debian:trixie-slim and set GOPROXY.
- Update Go version to 1.26 in Dockerfile and go.mod.
- Refactor token validation to use SHA-256 hashes instead of plain tokens.
- Add network policy to restrict access to the service.
- Enhance README with new configuration details and usage examples.
- Add tests for new token hash validation logic.

internal/auth/auth_test.go

@@ -1,11 +1,20 @@
 package auth
 
-import "testing"
+import (
+	"crypto/sha256"
+	"fmt"
+	"testing"
+)
+
+func hashHex(token string) string {
+	sum := sha256.Sum256([]byte(token))
+	return fmt.Sprintf("%x", sum[:])
+}
 
 func TestValidator_IsValid(t *testing.T) {
 	t.Parallel()
 
-	validator, err := NewValidator([]string{"token-one", "token-two", "token-three"})
+	validator, err := NewValidator([]string{hashHex("token-one"), hashHex("token-two"), hashHex("token-three")})
 	if err != nil {
 		t.Fatalf("NewValidator() error = %v", err)
 	}
@@ -45,6 +54,14 @@ func TestNewValidator_EmptyTokenSet(t *testing.T) {
 	}
 }
 
+func TestNewValidator_InvalidHash(t *testing.T) {
+	t.Parallel()
+
+	if _, err := NewValidator([]string{"not-a-hash"}); err == nil {
+		t.Fatal("NewValidator(invalid hash) expected error, got nil")
+	}
+}
+
 func TestParseBearerToken(t *testing.T) {
 	t.Parallel()