feat: add progress_callback to GVM process_sequence

[JamesNyeVRGuy]

Adds an optional progress_callback parameter to
GVMProcessor.process_sequence(). Called after each batch with (current_batch, total_batches). Backwards-compatible — defaults to None with no behavior change when not passed.

This enables callers (CLI, WebUI, etc.) to report per-batch progress during GVM alpha generation.

[shezmic]

This fixes an existing TypeError on upstream main.

backend/service.py (line 861) already calls:

gvm.process_sequence(
    ...
    progress_callback=_gvm_progress,
)

But gvm_core/wrapper.py's process_sequence() signature on main doesn't include progress_callback. Every call to service.run_gvm() currently throws:

TypeError: process_sequence() got an unexpected keyword argument 'progress_callback'

The implementation here is correct. The callback is called with (batch_id + 1, len(dataloader)), which matches the _gvm_progress(batch_idx, total_batches) contract in service.py. The None default preserves backward compatibility for any direct callers.

No no-go zones touched. Looks good to me.