fix: resolve critical production deployment issues

- Fix database connection error "getaddrinfo ENOTFOUND postgres"
- Add wait-for-db.sh script to ensure PostgreSQL is ready before starting
- Fix email processor initialization timing issue
- Add missing storage path environment variables
- Add database dependency to backend service
- Enhance health check endpoint with database connectivity check
- Update production database defaults to match docker-compose
- Install postgresql-client in Docker image for health checks
- Document all required environment variables in .env.example

Fixes immediate production deployment failures and ensures proper service startup order.

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

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

backend/.env.example

@@ -1,42 +1,41 @@
-NODE_ENV=development
+# Backend Environment Variables Example
+# Copy this file to .env and update with your values
+
+# Application
+NODE_ENV=production
 PORT=3000
 
-# URLs
-ADMIN_URL=http://localhost:3000
-FRONTEND_URL=http://localhost:3001
-
 # Security
-JWT_SECRET=dev-secret-key
+JWT_SECRET=your-very-secure-jwt-secret-at-least-32-characters-long
 
-# Email Configuration
-SMTP_HOST=mailhog
-SMTP_PORT=1025
-SMTP_SECURE=false
-SMTP_USER=
-SMTP_PASS=
-EMAIL_FROM=noreply@localhost
-
-# Storage Paths (relative to project root)
-STORAGE_PATH=./storage
-EVENTS_PATH=./storage/events
-ARCHIVE_PATH=./storage/events/archived
-
-# Logging
-LOG_LEVEL=info
+# URLs
+ADMIN_URL=https://yourdomain.com
+FRONTEND_URL=https://yourdomain.com
 
 # Database Configuration
-# Development: Use SQLite
-DATABASE_CLIENT=sqlite3
-DATABASE_PATH=./data/photo_sharing.db
+DATABASE_CLIENT=pg
+DB_HOST=db
+DB_PORT=5432
+DB_USER=picpeak
+DB_PASSWORD=your-secure-database-password
+DB_NAME=picpeak
 
-# Production: Use PostgreSQL
-# DATABASE_CLIENT=pg
-# DB_HOST=localhost
-# DB_PORT=5432
-# DB_USER=picpeak
-# DB_PASSWORD=your-secure-password
-# DB_NAME=picpeak
+# Email Configuration
+SMTP_HOST=smtp.example.com
+SMTP_PORT=587
+SMTP_SECURE=false
+SMTP_USER=your-smtp-username
+SMTP_PASS=your-smtp-password
+EMAIL_FROM=noreply@yourdomain.com
 
-# Umami Analytics (optional)
-UMAMI_URL=
-UMAMI_WEBSITE_ID=
+# Storage Paths (Docker)
+STORAGE_PATH=/app/storage
+EVENTS_PATH=/app/storage/events
+ARCHIVE_PATH=/app/storage/events/archived
+
+# Analytics (Optional)
+UMAMI_URL=https://analytics.yourdomain.com
+UMAMI_WEBSITE_ID=your-website-id
+
+# Logging
+LOG_LEVEL=info