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/workers/orchestrator.py

@@ -30,7 +30,7 @@ __package__ = 'archivebox.workers'
 import os
 import time
 from typing import Type
-from multiprocessing import Process
+from multiprocessing import Process as MPProcess
 
 from django.utils import timezone
 
@@ -38,12 +38,6 @@ from rich import print
 
 from archivebox.misc.logging_util import log_worker_event
 from .worker import Worker, CrawlWorker, SnapshotWorker, ArchiveResultWorker
-from .pid_utils import (
-    write_pid_file,
-    remove_pid_file,
-    get_all_worker_pids,
-    cleanup_stale_pid_files,
-)
 
 
 def _run_orchestrator_process(exit_on_idle: bool) -> None:
@@ -85,16 +79,20 @@ class Orchestrator:
     @classmethod
     def is_running(cls) -> bool:
         """Check if an orchestrator is already running."""
-        workers = get_all_worker_pids('orchestrator')
-        return len(workers) > 0
-    
+        from archivebox.machine.models import Process
+
+        return Process.get_running_count(process_type='orchestrator') > 0
+
     def on_startup(self) -> None:
         """Called when orchestrator starts."""
-        self.pid = os.getpid()
-        self.pid_file = write_pid_file('orchestrator', worker_id=0)
+        from archivebox.machine.models import Process
 
-        # Clean up any stale PID files from previous runs
-        stale_count = cleanup_stale_pid_files()
+        self.pid = os.getpid()
+        # Register orchestrator process in database
+        self.db_process = Process.current()
+
+        # Clean up any stale Process records from previous runs
+        stale_count = Process.cleanup_stale_running()
 
         # Collect startup metadata
         metadata = {
@@ -112,11 +110,15 @@ class Orchestrator:
             pid=self.pid,
             metadata=metadata,
         )
-    
+
     def on_shutdown(self, error: BaseException | None = None) -> None:
         """Called when orchestrator shuts down."""
-        if self.pid_file:
-            remove_pid_file(self.pid_file)
+        # Update Process record status
+        if hasattr(self, 'db_process') and self.db_process:
+            self.db_process.exit_code = 1 if error else 0
+            self.db_process.status = self.db_process.StatusChoices.EXITED
+            self.db_process.ended_at = timezone.now()
+            self.db_process.save()
 
         log_worker_event(
             worker_type='Orchestrator',
@@ -125,10 +127,12 @@ class Orchestrator:
             pid=self.pid,
             error=error if error and not isinstance(error, KeyboardInterrupt) else None,
         )
-    
+
     def get_total_worker_count(self) -> int:
         """Get total count of running workers across all types."""
-        cleanup_stale_pid_files()
+        from archivebox.machine.models import Process
+
+        Process.cleanup_stale_running()
         return sum(len(W.get_running_workers()) for W in self.WORKER_TYPES)
     
     def should_spawn_worker(self, WorkerClass: Type[Worker], queue_count: int) -> bool:
@@ -287,7 +291,7 @@ class Orchestrator:
         Returns the PID of the new process.
         """
         # Use module-level function to avoid pickle errors with local functions
-        proc = Process(
+        proc = MPProcess(
             target=_run_orchestrator_process,
             args=(self.exit_on_idle,),
             name='orchestrator'