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>

scripts/generate-jwt-secret.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+# Generate a secure JWT secret for PicPeak
+
+echo "==================================="
+echo "JWT Secret Generator for PicPeak"
+echo "==================================="
+echo ""
+
+# Generate the secret
+SECRET=$(openssl rand -hex 32)
+
+echo "Your new JWT secret (64 characters):"
+echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+echo "$SECRET"
+echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+echo ""
+echo "To use this secret:"
+echo ""
+echo "1. For Docker Compose (.env file):"
+echo "   JWT_SECRET=$SECRET"
+echo ""
+echo "2. For environment variable:"
+echo "   export JWT_SECRET=$SECRET"
+echo ""
+echo "3. For systemd service:"
+echo "   Environment=\"JWT_SECRET=$SECRET\""
+echo ""
+echo "⚠️  IMPORTANT:"
+echo "   - Keep this secret secure and never commit it to version control"
+echo "   - Use different secrets for different environments"
+echo "   - Store production secrets in a secure secret management system"
+echo "   - Rotate secrets regularly (every 90 days recommended)"
+echo ""