(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/management/commands/check_pobsync_install.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+from django.core.management.base import BaseCommand, CommandError
+
+from pobsync_backend.self_check import collect_self_checks, summarize_self_checks
+
+
+class Command(BaseCommand):
+    help = "Run pobsync runtime self checks for native installs and updates."
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            "--fail-on-warning",
+            action="store_true",
+            help="Exit with an error when warnings are present.",
+        )
+
+    def handle(self, *args, **options):
+        checks = collect_self_checks()
+        summary = summarize_self_checks(checks)
+
+        for check in checks:
+            line = f"[{check.status.upper()}] {check.name}: {check.message}"
+            if check.detail:
+                line = f"{line} ({check.detail})"
+            if check.status == "failed":
+                self.stderr.write(line)
+            elif check.status == "warning":
+                self.stderr.write(line)
+            else:
+                self.stdout.write(line)
+
+        self.stdout.write(
+            "Summary: "
+            f"{summary['ok']} ok, "
+            f"{summary['warning']} warning(s), "
+            f"{summary['failed']} failed, "
+            f"{summary['skipped']} skipped"
+        )
+
+        if summary["failed"]:
+            raise CommandError("pobsync install self check failed.")
+        if options["fail_on_warning"] and summary["warning"]:
+            raise CommandError("pobsync install self check reported warnings.")