chore: add MIT license and security policy

- Add MIT License for open source distribution
- Create SECURITY.md with vulnerability reporting process
- Include security best practices
- Document security features
- Professional open source project setup

Phase 4 Task 51/68 completed

LICENSE

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+MIT License
+
+Copyright (c) 2026 小虾 SaaS
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We release patches for security vulnerabilities in the following versions:
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 1.0.x   | :white_check_mark: |
+| < 1.0   | :x:                |
+
+## Reporting a Vulnerability
+
+We take the security of 小虾 SaaS seriously. If you believe you have found a security vulnerability, please report it to us as described below.
+
+### Please do NOT:
+
+- Open a public GitHub issue about the vulnerability
+- Discuss the vulnerability publicly (Twitter, blog posts, etc.)
+
+### Please DO:
+
+1. **Email us directly:** security@xiaoxia-saas.com
+2. **Include the following information:**
+   - Type of vulnerability
+   - Full path to the source file(s) related to the vulnerability
+   - Location of the affected code (tag/branch/commit)
+   - Step-by-step instructions to reproduce the issue
+   - Proof-of-concept or exploit code (if possible)
+   - Impact of the vulnerability
+
+### What to expect:
+
+- We will acknowledge your email within 48 hours
+- We will provide a more detailed response within 7 days
+- We will work on a fix and release a patch ASAP
+- We will credit you in the release notes (if you wish)
+
+## Security Best Practices
+
+When deploying 小虾 SaaS:
+
+1. **Change all default secrets:**
+   - `JWT_SECRET_KEY` (minimum 32 characters)
+   - Database passwords
+   - Redis passwords
+
+2. **Use HTTPS in production:**
+   - Configure SSL certificates
+   - Enable HTTPS redirect
+
+3. **Enable rate limiting:**
+   - Uncomment `RateLimitMiddleware` in production
+   - Configure appropriate limits
+
+4. **Regular updates:**
+   - Keep dependencies up to date
+   - Apply security patches promptly
+
+5. **Database security:**
+   - Use strong passwords
+   - Limit network access
+   - Enable SSL connections
+
+## Security Features
+
+小虾 SaaS includes:
+
+- ✅ bcrypt password hashing (cost=12)
+- ✅ JWT token signing and validation
+- ✅ SQL injection protection (parameterized queries)
+- ✅ XSS protection (input validation)
+- ✅ CORS configuration
+- ✅ Rate limiting
+- ✅ Session management
+
+## Disclosure Policy
+
+When we receive a security bug report, we will:
+
+1. Confirm the problem and determine affected versions
+2. Audit code to find similar problems
+3. Prepare fixes for all supported versions
+4. Release patches as soon as possible
+5. Publicly disclose the vulnerability
+
+Thank you for helping keep 小虾 SaaS and our users safe!