(feature) Add Django-managed SSH credentials

Add SSH credentials as first-class Django data so backup keys can be
uploaded through the control panel instead of mounted into containers.

Credentials can be selected globally or overridden per host. At runtime
the selected key is materialized inside the container with restrictive
file permissions and injected into the rsync SSH command via IdentityFile.
Known hosts entries are handled the same way when configured.

Add control panel views for creating and listing SSH keys, expose the
fields in config forms and admin, document the workflow, and cover global
and host credential selection with tests.

src/pobsync_backend/tests/test_django_config_source.py

@@ -1,9 +1,13 @@
 from __future__ import annotations
 
-from django.test import TestCase
+import stat
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from django.test import TestCase, override_settings
 
 from pobsync_backend.config_source import DjangoConfigSource
-from pobsync_backend.models import GlobalConfig, HostConfig
+from pobsync_backend.models import GlobalConfig, HostConfig, SshCredential
 
 
 class DjangoConfigSourceTests(TestCase):
@@ -58,3 +62,56 @@ class DjangoConfigSourceTests(TestCase):
         self.assertEqual(cfg["address"], "web-01.example.test")
         self.assertEqual(cfg["excludes_effective"], ["/proc/***", "/tmp/***"])
         self.assertEqual(cfg["rsync"]["args_effective"], ["--archive", "--numeric-ids", "--delete"])
+
+    def test_materializes_global_ssh_credential_for_runtime_config(self) -> None:
+        credential = SshCredential.objects.create(
+            name="backup-key",
+            private_key="PRIVATE KEY",
+            known_hosts="web-01.example.test ssh-ed25519 AAAATEST",
+        )
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/backups",
+            pobsync_home="/opt/pobsync",
+            default_ssh_credential=credential,
+            ssh_options=["-oBatchMode=yes"],
+        )
+        HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+        with TemporaryDirectory() as tmp, override_settings(POBSYNC_HOME=str(Path(tmp) / "home")):
+            cfg = DjangoConfigSource().effective_config_for_host("web-01")
+
+            identity_file = Path(tmp) / "home" / "state" / "ssh-credentials" / str(credential.pk) / "identity"
+            known_hosts = identity_file.parent / "known_hosts"
+            self.assertEqual(identity_file.read_text(encoding="utf-8"), "PRIVATE KEY\n")
+            self.assertEqual(known_hosts.read_text(encoding="utf-8"), "web-01.example.test ssh-ed25519 AAAATEST\n")
+            self.assertEqual(stat.S_IMODE(identity_file.stat().st_mode), 0o600)
+
+        self.assertIn("-oBatchMode=yes", cfg["ssh"]["options"])
+        self.assertIn(f"-oIdentityFile={identity_file}", cfg["ssh"]["options"])
+        self.assertIn(f"-oUserKnownHostsFile={known_hosts}", cfg["ssh"]["options"])
+
+    def test_host_ssh_credential_overrides_global_credential(self) -> None:
+        global_credential = SshCredential.objects.create(name="global-key", private_key="GLOBAL")
+        host_credential = SshCredential.objects.create(name="host-key", private_key="HOST")
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/backups",
+            pobsync_home="/opt/pobsync",
+            default_ssh_credential=global_credential,
+        )
+        HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            ssh_credential=host_credential,
+        )
+
+        with TemporaryDirectory() as tmp, override_settings(POBSYNC_HOME=str(Path(tmp) / "home")):
+            cfg = DjangoConfigSource().effective_config_for_host("web-01")
+
+            host_identity_file = Path(tmp) / "home" / "state" / "ssh-credentials" / str(host_credential.pk) / "identity"
+            global_identity_file = Path(tmp) / "home" / "state" / "ssh-credentials" / str(global_credential.pk) / "identity"
+            self.assertEqual(host_identity_file.read_text(encoding="utf-8"), "HOST\n")
+            self.assertFalse(global_identity_file.exists())
+
+        self.assertIn(f"-oIdentityFile={host_identity_file}", cfg["ssh"]["options"])