fix: convert empty date strings to NULL in migration

PostgreSQL DATE type doesn't accept empty strings ('').
Convert empty/whitespace date values to NULL during migration.

Fixes: invalid input syntax for type date: ""

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrate_to_postgres.py

@@ -90,6 +90,10 @@ def convert_value(value: Any, column: str, table: str) -> Any:
     if value is None:
         return None
 
+    # Empty string → NULL for DATE columns (PostgreSQL doesn't accept '' for DATE type)
+    if isinstance(value, str) and value.strip() == '' and column == 'date':
+        return None
+
     # INTEGER → BOOLEAN conversion
     if table in BOOLEAN_COLUMNS and column in BOOLEAN_COLUMNS[table]:
         return bool(value)