(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/forms.py

@@ -147,11 +147,16 @@ class ManualBackupForm(forms.Form):
 
 
 class SshCredentialForm(forms.ModelForm):
+    private_key_file = forms.FileField(
+        required=False,
+        help_text="Optional. Upload the private key file directly to avoid copy/paste formatting problems.",
+    )
     private_key = forms.CharField(
         widget=forms.Textarea(attrs={"spellcheck": "false", "autocomplete": "off"}),
+        required=False,
         help_text=(
             "Paste the complete unencrypted OpenSSH private key, including BEGIN/END lines. "
-            "Use the matching public key in the field below only as a cross-check."
+            "Leave empty when uploading a private key file."
         ),
     )
     public_key = forms.CharField(
@@ -171,7 +176,19 @@ class SshCredentialForm(forms.ModelForm):
         fields = ("name", "private_key", "public_key", "known_hosts", "notes")
 
     def clean_private_key(self) -> str:
-        private_key = normalize_private_key(self.cleaned_data["private_key"])
+        uploaded_file = self.files.get("private_key_file")
+        if uploaded_file:
+            try:
+                raw_private_key = uploaded_file.read().decode("utf-8")
+            except UnicodeDecodeError as exc:
+                raise forms.ValidationError("SSH private key files must be UTF-8 text files.") from exc
+        else:
+            raw_private_key = self.cleaned_data.get("private_key", "")
+
+        if not raw_private_key.strip():
+            raise forms.ValidationError("Paste a private key or upload a private key file.")
+
+        private_key = normalize_private_key(raw_private_key)
         public_key = validate_ssh_private_key(private_key)
         self.derived_public_key = public_key
         return f"{private_key}\n"