feat(build): linter for missing msgid definitions (#7109)

Add a new linter that checks that basic usages (those with an constant string) of the `Tr` function in Go and template files are referring to an existing translation value. Add it to the CI stack but not make it fail yet.

Signed-off-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7109
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>
Co-committed-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>

modules/locale/utils.go

@@ -0,0 +1,74 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// extracted from `/build/lint-locale.go`, `/build/lint-locale-usage.go`
+
+package locale
+
+import (
+	"encoding/json" //nolint:depguard
+	"fmt"
+
+	"gopkg.in/ini.v1" //nolint:depguard
+)
+
+func IterateMessagesContent(localeContent []byte, onMsgid func(string, string) error) error {
+	// Same configuration as Forgejo uses.
+	cfg := ini.Empty(ini.LoadOptions{
+		IgnoreContinuation: true,
+	})
+	cfg.NameMapper = ini.SnackCase
+
+	if err := cfg.Append(localeContent); err != nil {
+		return err
+	}
+
+	for _, section := range cfg.Sections() {
+		for _, key := range section.Keys() {
+			var trKey string
+			if section.Name() == "" || section.Name() == "DEFAULT" || section.Name() == "common" {
+				trKey = key.Name()
+			} else {
+				trKey = section.Name() + "." + key.Name()
+			}
+			if err := onMsgid(trKey, key.Value()); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
+func iterateMessagesNextInner(onMsgid func(string, string) error, data map[string]any, trKey ...string) error {
+	for key, value := range data {
+		currentKey := key
+		if len(trKey) == 1 {
+			currentKey = trKey[0] + "." + key
+		}
+
+		switch value := value.(type) {
+		case string:
+			if err := onMsgid(currentKey, value); err != nil {
+				return err
+			}
+		case map[string]any:
+			if err := iterateMessagesNextInner(onMsgid, value, currentKey); err != nil {
+				return err
+			}
+		default:
+			return fmt.Errorf("unexpected type: %s - %T", currentKey, value)
+		}
+	}
+
+	return nil
+}
+
+func IterateMessagesNextContent(localeContent []byte, onMsgid func(string, string) error) error {
+	var localeData map[string]any
+	if err := json.Unmarshal(localeContent, &localeData); err != nil {
+		return err
+	}
+	return iterateMessagesNextInner(onMsgid, localeData)
+}