fix: Reduce INSERT buffer size and update state after every flush

Problems identified:
1. Buffer size of batch_size * 10 (100k rows) was too large, causing
   migration_state to not update for several minutes on low-consolidation partitions
2. State updates only happened every 10 batches, not reflecting actual progress

Changes:
- Reduce insert_buffer_size from 10x to 5x batch_size (50k rows)
- Update migration_state after EVERY batch flush, not every 10 batches
- Add debug logging showing flush operations and total migrated count
- This provides better visibility into migration progress and checkpointing

For partitions with low consolidation ratio (like d0 with 1.1x), this ensures
migration_state is updated more frequently, supporting better resume capability
and providing visibility into actual progress.

🤖 Generated with Claude Code

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

src/migrator/full_migration.py

@@ -113,7 +113,8 @@ class FullMigrator:
 
                             # Accumulate rows for batch insertion to reduce database round-trips
                             insert_buffer = []
-                            insert_buffer_size = self.settings.migration.batch_size * 10  # Larger INSERT batches
+                            # Use smaller batch size for more frequent updates: batch_size * 5 = 50k rows
+                            insert_buffer_size = self.settings.migration.batch_size * 5
                             fetched_in_buffer = 0  # Track MySQL rows fetched (before consolidation)
 
                             # Fetch consolidation groups from partition
@@ -144,12 +145,14 @@ class FullMigrator:
                                         migrated += inserted
                                         batch_count += 1
                                         progress.update(fetched_in_buffer)
-
-                                        # Update state every 10 batches to track progress
-                                        if batch_count % 10 == 0:
-                                            self._update_migration_state(
-                                                pg_conn, migrated, None, migration_start_time
-                                            )
+                                        # Update migration state after every batch flush
+                                        self._update_migration_state(
+                                            pg_conn, migrated, None, migration_start_time
+                                        )
+                                        logger.debug(
+                                            f"Partition {partition}: flushed {inserted} rows, "
+                                            f"total migrated: {migrated}"
+                                        )
 
                                     insert_buffer = []
                                     fetched_in_buffer = 0
@@ -164,6 +167,10 @@ class FullMigrator:
                                     self._update_migration_state(
                                         pg_conn, migrated, None, migration_start_time
                                     )
+                                    logger.debug(
+                                        f"Partition {partition} final flush: {inserted} rows, "
+                                        f"total migrated: {migrated}"
+                                    )
 
                             logger.info(f"Partition {partition} complete: {partition_group_count} groups consolidated")