(bugfix) Validate Django-managed SSH private keys

Validate uploaded SSH private keys with ssh-keygen before saving them so
invalid, malformed, or unsupported key material is rejected in the
control panel instead of failing later during rsync.

Auto-populate the public key when it is omitted, add an edit flow for
existing SSH credentials, and cover create, update, and invalid-key
paths with view tests.

src/pobsync_backend/views.py

@@ -82,6 +82,29 @@ def create_ssh_credential(request):
         "pobsync_backend/ssh_credential_form.html",
         {
             "form": form,
+            "credential": None,
+        },
+    )
+
+
+@staff_member_required
+def edit_ssh_credential(request, credential_id: int):
+    credential = get_object_or_404(SshCredential, id=credential_id)
+    if request.method == "POST":
+        form = SshCredentialForm(request.POST, instance=credential)
+        if form.is_valid():
+            saved_credential = form.save()
+            messages.success(request, f"SSH credential saved for {saved_credential.name}.")
+            return redirect("ssh_credentials")
+    else:
+        form = SshCredentialForm(instance=credential)
+
+    return render(
+        request,
+        "pobsync_backend/ssh_credential_form.html",
+        {
+            "form": form,
+            "credential": credential,
         },
     )