(ops) Add terminal self-check command for native installs

Add check_pobsync_install so native deployments can run the same runtime
diagnostics from the terminal that are available in the Django Self Check view.

The command prints every check with status, returns a failing exit code when
install-critical checks fail, supports fail-on-warning for stricter automation,
and is documented in the installer output and README update flow.

src/pobsync_backend/tests/test_self_check.py

@@ -1,11 +1,14 @@
 from __future__ import annotations
 
 import subprocess
+from io import StringIO
 from unittest.mock import patch
 
+from django.core.management import call_command
+from django.core.management.base import CommandError
 from django.test import SimpleTestCase
 
-from pobsync_backend.self_check import _systemd_checks
+from pobsync_backend.self_check import SelfCheck, _systemd_checks
 
 
 class SystemdSelfCheckTests(SimpleTestCase):
@@ -40,3 +43,44 @@ class SystemdSelfCheckTests(SimpleTestCase):
         journal_check = next(check for check in checks if check.name == "Journal access")
         self.assertEqual(journal_check.status, "failed")
         self.assertEqual(journal_check.message, "pobsync cannot read service logs.")
+
+
+class CheckPobsyncInstallCommandTests(SimpleTestCase):
+    def test_command_prints_summary_for_successful_checks(self) -> None:
+        stdout = StringIO()
+        stderr = StringIO()
+        checks = [
+            SelfCheck("Database connection", "ok", "django.db.backends.sqlite3"),
+            SelfCheck("Systemd services", "skipped", "systemd is not available in this runtime."),
+        ]
+
+        with patch("pobsync_backend.management.commands.check_pobsync_install.collect_self_checks", return_value=checks):
+            call_command("check_pobsync_install", stdout=stdout, stderr=stderr)
+
+        self.assertIn("[OK] Database connection", stdout.getvalue())
+        self.assertIn("[SKIPPED] Systemd services", stdout.getvalue())
+        self.assertIn("Summary: 1 ok, 0 warning(s), 0 failed, 1 skipped", stdout.getvalue())
+        self.assertEqual(stderr.getvalue(), "")
+
+    def test_command_fails_when_checks_fail(self) -> None:
+        stdout = StringIO()
+        stderr = StringIO()
+        checks = [
+            SelfCheck("POBSYNC_BACKUP_ROOT", "failed", "/backups does not exist."),
+        ]
+
+        with patch("pobsync_backend.management.commands.check_pobsync_install.collect_self_checks", return_value=checks):
+            with self.assertRaisesMessage(CommandError, "pobsync install self check failed."):
+                call_command("check_pobsync_install", stdout=stdout, stderr=stderr)
+
+        self.assertIn("[FAILED] POBSYNC_BACKUP_ROOT", stderr.getvalue())
+        self.assertIn("Summary: 0 ok, 0 warning(s), 1 failed, 0 skipped", stdout.getvalue())
+
+    def test_command_can_fail_on_warnings(self) -> None:
+        checks = [
+            SelfCheck("Global config", "warning", "Default global config has not been created yet."),
+        ]
+
+        with patch("pobsync_backend.management.commands.check_pobsync_install.collect_self_checks", return_value=checks):
+            with self.assertRaisesMessage(CommandError, "pobsync install self check reported warnings."):
+                call_command("check_pobsync_install", "--fail-on-warning", stdout=StringIO(), stderr=StringIO())