(feature) Add host key scanning for SSH credentials

Add a host detail action that scans the target SSH host key with
ssh-keyscan and stores it on the selected SSH credential.

Merge scanned known_hosts entries without duplicates and let the
existing runtime config pass them through as UserKnownHostsFile for
unattended rsync over SSH.

Extend host checks to warn when the selected credential has no known_hosts
entries, making host key verification failures actionable from Django.

src/pobsync_backend/tests/test_ssh_credentials.py

@@ -10,6 +10,7 @@ from django.test import SimpleTestCase, TestCase, override_settings
 
 from pobsync_backend.forms import normalize_private_key, validate_ssh_private_key
 from pobsync_backend.models import GlobalConfig, SshCredential
+from pobsync_backend.ssh_keys import merge_known_hosts
 
 
 class SshCredentialValidationTests(SimpleTestCase):
@@ -60,3 +61,14 @@ class SshCredentialManagementTests(TestCase):
 
         global_config.refresh_from_db()
         self.assertEqual(global_config.default_ssh_credential.name, "default")
+
+    def test_merge_known_hosts_appends_unique_entries(self) -> None:
+        merged = merge_known_hosts(
+            "web-01.example.test ssh-ed25519 AAAAOLD\n",
+            "web-01.example.test ssh-ed25519 AAAAOLD\nweb-01.example.test ssh-rsa AAAANEW\n",
+        )
+
+        self.assertEqual(
+            merged,
+            "web-01.example.test ssh-ed25519 AAAAOLD\nweb-01.example.test ssh-rsa AAAANEW\n",
+        )