fix: Allow dry-run even when migration is in progress

Problem: dry-run was blocked with 'Migration already in progress' error,
even though dry-run should not require --resume (it doesn't modify data).

Fix: Skip the 'already in progress' check if dry_run is True. This allows:
- Testing partitions without interrupting active migration
- Verifying what would be migrated without needing --resume
- Checking consolidation logic on specific partitions

Also improved dry-run message to show partition info if specified.

🤖 Generated with Claude Code

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

src/migrator/full_migration.py

@@ -78,11 +78,12 @@ class FullMigrator:
                         logger.warning(
                             f"Found previous migration state: {pg_row_count} rows already in {pg_table}"
                         )
-                        if not resume:
+                        if not resume and not dry_run:
                             raise ValueError(
                                 f"Migration already in progress for {pg_table}. "
                                 f"Use --resume to continue from last checkpoint, or delete data to restart."
                             )
+                        if resume:
                             logger.info(f"Resuming migration - found {pg_row_count} existing rows")
                             if last_completed_partition:
                                 logger.info(f"Last completed partition: {last_completed_partition}")
@@ -95,6 +96,9 @@ class FullMigrator:
                         rows_to_migrate = total_rows
 
                     if dry_run:
+                        if partition:
+                            logger.info(f"[DRY RUN] Would migrate partition {partition} (~{rows_to_migrate} total rows in table)")
+                        else:
                             logger.info(f"[DRY RUN] Would migrate {rows_to_migrate} rows")
                         return rows_to_migrate