Fix code review issues in process management refactor

- Add pwd validation in Process.launch() to prevent crashes
- Fix psutil returncode handling (use wait() return value, not returncode attr)
- Add None check for proc.pid in cleanup_stale_running()
- Add stale process cleanup in Orchestrator.is_running()
- Ensure orchestrator process_type is correctly set to ORCHESTRATOR
- Fix KeyboardInterrupt handling (exit code 0 for graceful shutdown)
- Throttle cleanup_stale_running() to once per 30 seconds for performance
- Fix worker process_type to use TypeChoices.WORKER consistently
- Fix get_running_workers() API to return list of dicts (not Process objects)
- Only delete PID files after successful kill or confirmed stale
- Fix migration index names to match between SQL and Django state
- Remove db_index=True from process_type (index created manually)
- Update documentation to reflect actual implementation
- Add explanatory comments to empty except blocks
- Fix exit codes to use Unix convention (128 + signal number)

Co-authored-by: Nick Sweeting <pirate@users.noreply.github.com>

archivebox/machine/migrations/0002_process_parent_and_type.py

@@ -70,7 +70,6 @@ class Migration(migrations.Migration):
                             ('hook', 'Hook Script'),
                             ('binary', 'Binary Execution'),
                         ],
-                        db_index=True,
                         default='binary',
                         help_text='Type of process in the execution hierarchy',
                         max_length=16,
@@ -81,14 +80,14 @@ class Migration(migrations.Migration):
                     model_name='process',
                     index=models.Index(
                         fields=['parent', 'status'],
-                        name='machine_pro_parent__status_idx',
+                        name='machine_process_parent_status_idx',
                     ),
                 ),
                 migrations.AddIndex(
                     model_name='process',
                     index=models.Index(
                         fields=['machine', 'pid', 'started_at'],
-                        name='machine_pro_machine_pid_idx',
+                        name='machine_process_machine_pid_started_idx',
                     ),
                 ),
             ],