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>

docker-compose.yml

@@ -112,6 +112,12 @@ services:
       SECRET_KEY: ${SECRET_KEY}
       DATABASE_URL: postgresql://${DB_USER:-keepitgoing}:${DB_PASSWORD}@postgres:5432/${DB_NAME:-keepitgoing}
       REDIS_URL: redis://redis:6379/0
+      EMAIL_HOST: ${EMAIL_HOST:-}
+      EMAIL_PORT: ${EMAIL_PORT:-587}
+      EMAIL_HOST_USER: ${EMAIL_HOST_USER:-}
+      EMAIL_HOST_PASSWORD: ${EMAIL_HOST_PASSWORD:-}
+      EMAIL_USE_TLS: ${EMAIL_USE_TLS:-True}
+      DEFAULT_FROM_EMAIL: ${DEFAULT_FROM_EMAIL:-noreply@localhost}
     networks:
       - backend
     depends_on:
@@ -120,6 +126,12 @@ services:
       redis:
         condition: service_healthy
     command: celery-worker
+    healthcheck:
+      test: ["CMD-SHELL", "celery -A config inspect ping -d celery@$$HOSTNAME"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 30s
 
   # =================================================================
   # Celery Beat Service - Scheduled task scheduler
@@ -135,6 +147,12 @@ services:
       SECRET_KEY: ${SECRET_KEY}
       DATABASE_URL: postgresql://${DB_USER:-keepitgoing}:${DB_PASSWORD}@postgres:5432/${DB_NAME:-keepitgoing}
       REDIS_URL: redis://redis:6379/0
+      EMAIL_HOST: ${EMAIL_HOST:-}
+      EMAIL_PORT: ${EMAIL_PORT:-587}
+      EMAIL_HOST_USER: ${EMAIL_HOST_USER:-}
+      EMAIL_HOST_PASSWORD: ${EMAIL_HOST_PASSWORD:-}
+      EMAIL_USE_TLS: ${EMAIL_USE_TLS:-True}
+      DEFAULT_FROM_EMAIL: ${DEFAULT_FROM_EMAIL:-noreply@localhost}
     networks:
       - backend
     depends_on:
@@ -143,6 +161,12 @@ services:
       redis:
         condition: service_healthy
     command: celery-beat
+    healthcheck:
+      test: ["CMD-SHELL", "pgrep -f 'celery.*beat' || exit 1"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 40s
 
 # =================================================================
 # Named Volumes