feat: Add audit logging (#38189)

Co-authored-by: bircni <bircni@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/audit/message.go

@@ -0,0 +1,66 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package audit
+
+import (
+	"fmt"
+	"strings"
+
+	audit_model "gitea.dev/models/audit"
+	"gitea.dev/modules/log"
+)
+
+// Reserved placeholders, filled from the event itself rather than from metadata.
+const (
+	placeholderScope = "scope"
+	placeholderActor = "actor"
+)
+
+// renderMessage fills the action's template from the event's scope, actor and
+// metadata. A missing template or an unresolved placeholder is logged and
+// rendered as the bare key: audit recording must never fail the request that
+// triggered it, and a partial message is more useful than none.
+func renderMessage(action audit_model.Action, actor, scope audit_model.EntityRef, metadata map[string]any) string {
+	tmpl, ok := audit_model.MessageTemplate(action)
+	if !ok {
+		log.Error("audit: no message template for action %q", action)
+		return string(action)
+	}
+
+	var sb strings.Builder
+	rest := tmpl
+	for {
+		start := strings.IndexByte(rest, '{')
+		if start < 0 {
+			break
+		}
+		end := strings.IndexByte(rest[start:], '}')
+		if end < 0 {
+			break
+		}
+		end += start
+
+		key := rest[start+1 : end]
+		sb.WriteString(rest[:start])
+		sb.WriteString(resolvePlaceholder(action, key, actor, scope, metadata))
+		rest = rest[end+1:]
+	}
+	sb.WriteString(rest)
+
+	return sb.String()
+}
+
+func resolvePlaceholder(action audit_model.Action, key string, actor, scope audit_model.EntityRef, metadata map[string]any) string {
+	switch key {
+	case placeholderScope:
+		return scope.DisplayName()
+	case placeholderActor:
+		return actor.DisplayName()
+	}
+	if v, ok := metadata[key]; ok {
+		return fmt.Sprint(v)
+	}
+	log.Error("audit: action %q has no metadata for placeholder %q", action, key)
+	return key
+}