(bugfix) Surface rsync SSH failure details in run results

Include the selected SSH credential metadata and rsync log tail in
dry-run and failed backup results so Django shows the actual SSH or
rsync failure instead of only the exit code.

Warn in host checks when a host still uses database-stored private key
material, making it easier to spot old credentials after switching to
generated filesystem keys.

src/pobsync_backend/host_ops.py

@@ -53,6 +53,15 @@ def collect_host_checks(host: HostConfig, global_config: GlobalConfig | None = N
             checks.append(
                 _host_path_check("Host SSH key file", key_path, must_exist=True, must_be_writable=False, must_be_readable=True)
             )
+        elif credential.private_key:
+            checks.append(
+                SelfCheck(
+                    "Host SSH key storage",
+                    "warning",
+                    "Selected credential stores private key material in the database.",
+                    "Generated filesystem keys are recommended for native systemd installs.",
+                )
+            )
 
     host_root = resolve_host_root(global_config.backup_root, host.host)
     checks.append(_host_path_check("Host backup root", host_root, must_exist=True, must_be_writable=True))