feat: Add periodic throughput reporting to progress tracker

For very large migrations (111M rows), the progress bar can appear frozen
when showing percentage-based progress on 60M+ remaining rows. Even at
20k rows/sec, progress moves slowly on screen.

Solution: Print periodic throughput updates every 1M rows processed.
Shows:
- Actual count processed and total
- Current throughput in rows/sec
- Elapsed time in hours

This gives users visual feedback that migration is actively processing
without needing to wait for percentage to visibly change.

Example output:
  Progress: 5,000,000/111,000,000 items (18,500 items/sec, 4.2h elapsed)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/utils/progress.py

@@ -67,6 +67,17 @@ class ProgressTracker:
             self.progress.update(self.task_id, advance=advance)
             self.processed += advance
 
+            # For very large totals (e.g., migrations), show throughput periodically
+            # to give visual feedback even when progress percentage seems stuck
+            if self.processed % 1000000 == 0 and self.processed > 0:
+                elapsed = time.time() - self.start_time
+                if elapsed > 0:
+                    rate = self.processed / elapsed
+                    self.progress.print(
+                        f"[cyan]Progress: {self.processed:,}/{self.total:,} items "
+                        f"({rate:.0f} items/sec, {elapsed/3600:.1f}h elapsed)[/cyan]"
+                    )
+
     def print_status(self, message: str):
         """Print a status message without interrupting progress bar."""
         if self.task_id is not None: