Update docker-compose.yml with all environment variables and PostgreSQL

Major changes:
- Switch from MariaDB to PostgreSQL 16
- Add all required environment variables to web service:
  - SECRET_KEY, DEBUG, ALLOWED_HOSTS
  - CSRF_TRUSTED_ORIGINS, CORS_ALLOWED_ORIGINS
  - Email configuration (HOST, PORT, USER, PASSWORD, TLS)
  - Gunicorn settings (WORKERS, TIMEOUT)
- Add SECRET_KEY to celery services (required for Django)
- Update all services to depend on 'postgres' instead of 'mariadb'
- Update DATABASE_URL to use PostgreSQL format
- Update .env.docker.example to reflect PostgreSQL and add WEB_PORT

All environment variables now properly declared in compose file
with ${VAR} syntax for Portainer/Docker compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.env.docker.example

@@ -27,15 +27,12 @@ CSRF_TRUSTED_ORIGINS=https://yourdomain.com
 SITE_DOMAIN=yourdomain.com
 
 # ===================================================================
-# Database Configuration (MariaDB)
+# Database Configuration (PostgreSQL)
 # ===================================================================
 
 DB_NAME=keepitgoing
 DB_USER=keepitgoing
 DB_PASSWORD=CHANGE_ME_STRONG_DATABASE_PASSWORD
-DB_ROOT_PASSWORD=CHANGE_ME_STRONG_ROOT_PASSWORD
-DB_HOST=mariadb
-DB_PORT=3306
 
 # ===================================================================
 # Redis Configuration
@@ -101,6 +98,14 @@ CORS_ALLOWED_ORIGINS=https://yourdomain.com
 
 SAAS_MODE=False
 
+# ===================================================================
+# Web Server Configuration
+# ===================================================================
+
+# Port to expose the web service on (default: 8000)
+# Change this if port 8000 is already in use
+WEB_PORT=8000
+
 # ===================================================================
 # Gunicorn Configuration
 # ===================================================================