Delete pid_utils.py and migrate to Process model

DELETED:
- workers/pid_utils.py (-192 lines) - replaced by Process model methods

SIMPLIFIED:
- crawls/models.py Crawl.cleanup() (80 lines -> 10 lines)
- hooks.py: deleted process_is_alive() and kill_process() (-45 lines)

UPDATED to use Process model:
- core/models.py: Snapshot.cleanup() and has_running_background_hooks()
- machine/models.py: Binary.cleanup()
- workers/worker.py: Worker.on_startup/shutdown, get_running_workers, start
- workers/orchestrator.py: Orchestrator.on_startup/shutdown, is_running

All subprocess management now uses:
- Process.current() for registering current process
- Process.get_running() / get_running_count() for querying
- Process.cleanup_stale_running() for cleanup
- safe_kill_process() for validated PID killing

Total line reduction: ~250 lines

archivebox/hooks.py

@@ -1233,52 +1233,3 @@ def process_hook_records(records: List[Dict[str, Any]], overrides: Dict[str, Any
             continue
 
     return stats
-
-
-def process_is_alive(pid_file: Path) -> bool:
-    """
-    Check if process in PID file is still running.
-
-    Args:
-        pid_file: Path to hook.pid file
-
-    Returns:
-        True if process is alive, False otherwise
-    """
-    if not pid_file.exists():
-        return False
-
-    try:
-        pid = int(pid_file.read_text().strip())
-        os.kill(pid, 0)  # Signal 0 = check if process exists without killing it
-        return True
-    except (OSError, ValueError):
-        return False
-
-
-def kill_process(pid_file: Path, sig: int = signal.SIGTERM, validate: bool = True):
-    """
-    Kill process in PID file with optional validation.
-
-    Args:
-        pid_file: Path to hook.pid file
-        sig: Signal to send (default SIGTERM)
-        validate: If True, validate process identity before killing (default: True)
-    """
-    from archivebox.misc.process_utils import safe_kill_process
-    
-    if validate:
-        # Use safe kill with validation
-        cmd_file = pid_file.parent / 'cmd.sh'
-        safe_kill_process(pid_file, cmd_file, signal_num=sig)
-    else:
-        # Legacy behavior - kill without validation
-        if not pid_file.exists():
-            return
-        try:
-            pid = int(pid_file.read_text().strip())
-            os.kill(pid, sig)
-        except (OSError, ValueError):
-            pass
-
-