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 <[email protected]>

backend/knexfile.js

@@ -27,11 +27,11 @@ const config = {
   production: {
     client: process.env.DATABASE_CLIENT || 'pg',
     connection: {
-      host: process.env.DB_HOST || 'postgres',
+      host: process.env.DB_HOST || 'db',
       port: process.env.DB_PORT || 5432,
-      user: process.env.DB_USER || 'postgres',
-      password: process.env.DB_PASSWORD || 'postgres',
-      database: process.env.DB_NAME || 'photo_sharing'
+      user: process.env.DB_USER || 'picpeak',
+      password: process.env.DB_PASSWORD,
+      database: process.env.DB_NAME || 'picpeak'
     },
     pool: {
       min: 2,