docs: add warnings about $ character in Docker Compose passwords

- Add clear warnings in .env.example about $ variable substitution
- Update DEPLOYMENT_GUIDE.md with password generation commands that exclude $
- Add troubleshooting section for Docker Compose variable substitution errors
- Provide solutions: avoid $, escape as $$, or use quotes

Fixes issue where passwords containing $ cause Docker Compose warnings
and potential authentication failures.

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

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

.env.example

@@ -10,10 +10,13 @@ JWT_SECRET=your_very_long_random_jwt_secret_here
 # Database Configuration (PostgreSQL)
 DATABASE_CLIENT=pg
 DB_USER=picpeak
+# IMPORTANT: Avoid $ character in passwords - Docker Compose interprets it as variable substitution
+# If you must use $, escape it as $$ (e.g., Pass$$word instead of Pass$word)
 DB_PASSWORD=your_secure_postgres_password_here
 DB_NAME=picpeak_prod
 
 # Redis Configuration
+# IMPORTANT: Same warning applies - avoid $ or escape as $$
 REDIS_PASSWORD=your_secure_redis_password_here
 
 # Admin Account (initial setup)