🔒 Security: Remove hardcoded credentials and enforce environment variable validation

[Copilot]

Problem

This PR addresses a critical security vulnerability where sensitive credentials were hardcoded in the application source code:

• JWT Secret Key: SECRET_KEY = "your-secret-key-for-development"
• Database Password: POSTGRES_PASSWORD = "123456"
• Initial Admin Credentials: FIRST_SUPERUSER = "admin", FIRST_SUPERUSER_PASSWORD = "admin"

These hardcoded values made it trivial for attackers to compromise both development and potentially production environments.

Solution

🛡️ Implemented Security Validation System

Added a comprehensive environment variable validation system that:

• Checks for all required security-sensitive environment variables at application startup
• Provides clear, actionable error messages when variables are missing
• Forces explicit configuration of all sensitive credentials
• Prevents the application from starting with insecure defaults

🔧 Removed All Hardcoded Credentials

Before:

SECRET_KEY: str = os.getenv("SECRET_KEY", "your-secret-key-for-development")
POSTGRES_PASSWORD: str = os.getenv("POSTGRES_PASSWORD", "123456")
# Hardcoded fallback database URI
return "postgresql://postgres:123456@localhost:5432/GradNote"

After:

SECRET_KEY: str = os.getenv("SECRET_KEY", "")
POSTGRES_PASSWORD: str = os.getenv("POSTGRES_PASSWORD", "")
# Validates all required fields, no unsafe fallbacks
raise ValueError(f"数据库配置不完整：缺少 {key} 环境变量")

📝 Enhanced Documentation

Updated .env.example with comprehensive security guidance:

• Clear sections for required vs optional configuration
• Security warnings about using strong passwords
• Suggestions for secure key generation (openssl rand -hex 32)
• Explicit documentation of all required environment variables

Security Impact

• Before: Credentials visible in source code, unsafe defaults used in production
• After: All credentials must be explicitly configured, application refuses to start without proper security configuration

Required Environment Variables

The following variables are now required and must be set before the application will start:

• SECRET_KEY: JWT signing key (recommend: openssl rand -hex 32)
• POSTGRES_PASSWORD: Database password
• FIRST_SUPERUSER: Initial admin username
• FIRST_SUPERUSER_PASSWORD: Initial admin password
• FIRST_SUPERUSER_EMAIL: Initial admin email

Testing

Comprehensive test suite validates:

• ✅ Application correctly rejects startup when environment variables are missing
• ✅ Clear error messages guide users to proper configuration
• ✅ Application functions normally when all required variables are provided
• ✅ No hardcoded credentials remain in source code
• ✅ Database configuration validation works correctly

Breaking Change Notice

⚠️ This is a breaking change for deployments. After this update:

1. Set all required environment variables in your deployment
2. Reference the updated .env.example for guidance
3. Ensure your deployment scripts/containers provide all required variables

The application will fail to start with clear error messages if any required environment variables are missing, preventing accidental deployment with insecure defaults.

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