(bugfix) Add SSH private key file upload

Allow SSH credentials to be created from an uploaded private key file
as an alternative to pasting the key into a textarea.

Use multipart form handling in the credential views so server-side
keys can be imported without copy/paste wrapping or formatting damage.

Cover the upload path with a view test while keeping existing pasted
key validation behavior intact.

src/pobsync_backend/views.py

@@ -83,7 +83,7 @@ def ssh_credentials(request):
 @staff_member_required
 def create_ssh_credential(request):
     if request.method == "POST":
-        form = SshCredentialForm(request.POST)
+        form = SshCredentialForm(request.POST, request.FILES)
         if form.is_valid():
             credential = form.save()
             messages.success(request, f"SSH credential saved for {credential.name}.")
@@ -105,7 +105,7 @@ def create_ssh_credential(request):
 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)
+        form = SshCredentialForm(request.POST, request.FILES, instance=credential)
         if form.is_valid():
             saved_credential = form.save()
             messages.success(request, f"SSH credential saved for {saved_credential.name}.")