🧑‍💻(admin) add mailbox-related models to django admin

Register MailDomain, MailDomainAccess and Mailbox to django admin.

src/backend/mailbox_manager/admin.py

@@ -0,0 +1,33 @@
+"""Admin classes and registrations for People's mailbox manager app."""
+
+from django.contrib import admin
+from django.utils.translation import gettext_lazy as _
+
+from mailbox_manager import models
+
+
+@admin.register(models.MailDomain)
+class MailDomainAdmin(admin.ModelAdmin):
+    """Mail domain admin interface declaration."""
+
+    list_display = (
+        "name",
+        "created_at",
+        "updated_at",
+    )
+    search_fields = ("name",)
+    readonly_fields = ["created_at"]
+
+
+@admin.register(models.MailDomainAccess)
+class MailDomainAccessAdmin(admin.ModelAdmin):
+    """Admin for mail domain accesses model."""
+
+    list_display = ("user", "domain")
+
+
+@admin.register(models.Mailbox)
+class MailboxAdmin(admin.ModelAdmin):
+    """Admin for mailbox model."""
+
+    list_display = ("__str__", "domain")