Fix celery container health issues

- Fix import error in notifications/tasks.py (timezone.datetime -> datetime)
- Add healthchecks to celery-worker and celery-beat containers
- Add procps package to Dockerfile for pgrep command
- Add email environment variables to celery containers

The import error was causing celery workers to crash when loading tasks.
Missing healthchecks prevented proper container health monitoring.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Dockerfile

@@ -28,6 +28,7 @@ FROM python:3.12-slim-bookworm
 RUN apt-get update && apt-get install -y --no-install-recommends \
     postgresql-client \
     curl \
+    procps \
     && rm -rf /var/lib/apt/lists/*
 
 # Create django user (non-root for security)