(ui) Remove legacy-facing labels from operator pages

Replace refactor-era wording such as Source, Source root, SQL records,
database, runtime, and Django generation labels with operator-facing copy
around backup source, tracking records, changelog files, active config, and
pobsync-managed SSH keys.

Add view assertions so the old source/SQL labels do not quietly return.

Refs #24

src/pobsync_backend/forms.py

@@ -192,7 +192,7 @@ class SshCredentialForm(forms.ModelForm):
         if not raw_private_key.strip():
             if self.instance and self.instance.pk and self.instance.key_path:
                 return self.instance.private_key
-            raise forms.ValidationError("Paste a private key, upload a private key file, or generate a key from Django.")
+            raise forms.ValidationError("Paste a private key, upload a private key file, or generate a key in pobsync.")
 
         private_key = normalize_private_key(raw_private_key)
         public_key = validate_ssh_private_key(private_key)