fix: pin bcrypt==4.0.1 for passlib compat, explicit celery task imports

- bcrypt 5.x breaks passlib 1.7.4, pin to 4.0.1
- autodiscover_tasks not finding task modules, add explicit imports

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/tasks/celery_app.py

@@ -25,3 +25,7 @@ celery_app.conf.update(
 )
 
 celery_app.autodiscover_tasks(["app.tasks"])
+
+import app.tasks.transcription  # noqa: F401, E402
+import app.tasks.protocol  # noqa: F401, E402
+import app.tasks.cleanup  # noqa: F401, E402

backend/requirements.txt

@@ -15,6 +15,7 @@ pydantic-settings==2.7.1
 
 # Auth
 passlib[bcrypt]==1.7.4
+bcrypt==4.0.1
 python-jose[cryptography]==3.3.0
 
 # Task queue