docs: add security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues, discussions, or pull requests.**
+
+Instead, report them privately via GitHub's [private vulnerability reporting](https://github.com/developmentseed/stac-auth-proxy/security/advisories/new) (the "Report a vulnerability" button on the repository's [Security tab](https://github.com/developmentseed/stac-auth-proxy/security)).
+
+Please include as much of the following as you can:
+
+- A description of the issue and its impact (e.g. authentication bypass, filter bypass, data exposure)
+- Steps to reproduce, ideally with a minimal configuration and example requests
+- Affected version(s) and relevant configuration (e.g. enabled filters, upstream STAC API)
+- Any suggested fix or mitigation
+
+## What to Expect
+
+- We aim to acknowledge reports within **5 business days**.
+- We will keep you informed as we investigate and work on a fix.
+- Once a fix is released, we will publish a [GitHub Security Advisory](https://github.com/developmentseed/stac-auth-proxy/security/advisories) and, where appropriate, request a CVE. Reporters are credited unless they prefer to remain anonymous.
+
+Please give us a reasonable amount of time to address the issue before any public disclosure.
+
+## Supported Versions
+
+Security fixes are applied to the latest release only. Please upgrade to the most recent version before reporting.