ldap: Implement external auth id auth+sync.

Fixes #24104.

zerver/migrations/0729_externalauthid.py

@@ -0,0 +1,49 @@
+# Generated by Django 5.2.3 on 2025-07-08 00:35
+
+import django.db.models.deletion
+import django.utils.timezone
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0728_alter_stream_can_resolve_topics_group"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="ExternalAuthID",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True, primary_key=True, serialize=False, verbose_name="ID"
+                    ),
+                ),
+                ("date_created", models.DateTimeField(default=django.utils.timezone.now)),
+                ("external_auth_method_name", models.TextField()),
+                ("external_auth_id", models.TextField()),
+                (
+                    "realm",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE, to="zerver.realm"
+                    ),
+                ),
+                (
+                    "user",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL
+                    ),
+                ),
+            ],
+            options={
+                "constraints": [
+                    models.UniqueConstraint(
+                        fields=("realm", "external_auth_method_name", "external_auth_id"),
+                        name="zerver_externalauthid_uniq",
+                    )
+                ],
+            },
+        ),
+    ]