feat(security): implement secure JWT secrets for all environments

- Replace short dev secrets with secure 64-character (256-bit) secrets
- Update docker-compose.yml with secure development secret
- Update docker-compose.local.yml with unique secure secret
- Improve .env.example with clear security instructions
- Add comprehensive security best practices documentation
- Create helper script to generate secure JWT secrets

Security improvements:
- All environments now use cryptographically secure 64-character secrets
- Clear warnings and instructions prevent use of weak secrets
- Documentation guides proper secret management
- Helper script makes it easy to generate new secrets

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

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

.env.example

@@ -1,5 +1,7 @@
 # JWT Secret for authentication
-JWT_SECRET=your-secret-key-here
+# IMPORTANT: Generate a secure random secret with: openssl rand -hex 32
+# NEVER use the default value or commit the actual secret to version control
+JWT_SECRET=CHANGE_ME_TO_A_64_CHARACTER_SECURE_RANDOM_STRING_GENERATED_BY_OPENSSL
 
 # URLs
 ADMIN_URL=https://admin.photos.yourdomain.com