fix(status): prevent false Completed status when mandatory pipeline steps have not run

Add a terminal-step guard (send_to_all_destinations) to all status
calculation paths so that files are only marked Completed once the
entire processing pipeline has been recorded.

- get_file_overall_status: require TERMINAL_STEP to be present
- get_files_processing_status: same guard for bulk status
- get_step_summary: count missing terminal step as queued so
  total_main_steps > main_completed when pipeline is incomplete
- apply_status_filter: SQL sub-query requires terminal step for
  completed filter
- process_document: call initialize_file_steps after creating a new
  file record so all mandatory steps are pre-created as pending

Define TERMINAL_STEP constant in step_manager.py and reference it in
file_status.py and file_queries.py to avoid magic strings.

Tests updated: add send_to_all_destinations to completed-file
fixtures; add test verifying initialize_file_steps is called for
new files.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_file_status_fix.py

@@ -437,7 +437,7 @@ class TestFileStatusMissingCoverage:
         assert result["has_errors"] is False
 
     def test_get_files_processing_status_with_completed_steps(self, db_session):
-        """Covers line 189->188: completed + skipped == total_steps → completed status."""
+        """Covers completed + skipped == total_steps with terminal step → completed status."""
         from datetime import datetime
         from unittest.mock import patch
 
@@ -457,6 +457,7 @@ class TestFileStatusMissingCoverage:
         for step_name, step_status in [
             ("create_file_record", "success"),
             ("finalize_document_storage", "skipped"),
+            ("send_to_all_destinations", "success"),
         ]:
             step = FileProcessingStep(
                 file_id=file_record.id,