docs: add SECURITY.md security policy

SECURITY.md

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+# Security Policy
+
+PostGuard for Business handles identity attributes, email signing, and API
+keys, so we take security reports seriously and appreciate responsible
+disclosure.
+
+## Reporting a vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues,
+pull requests, or discussions.**
+
+Instead, use one of these private channels:
+
+1. **GitHub private vulnerability reporting (preferred).** Open a report via the
+   repository's **Security** tab → **Report a vulnerability**, or directly at
+   <https://github.com/encryption4all/postguard-business/security/advisories/new>.
+2. **Email (fallback).** Send the details to **info@postguard.eu**.
+
+Please include as much of the following as you can:
+
+- A description of the vulnerability and its potential impact.
+- Steps to reproduce, or a proof of concept.
+- The affected version, tag, commit, or URL.
+- Any suggested remediation, if you have one.
+
+## What to expect
+
+- We aim to acknowledge a report within **5 business days**.
+- We will keep you informed of progress toward a fix and coordinate on
+  disclosure timing.
+- With your consent, we are happy to credit you once the issue is resolved.
+
+## Supported versions
+
+This is a continuously released application. Security fixes land on `main` and
+are shipped in the latest release; older tags do not receive backported fixes.
+
+| Version                 | Supported |
+| ----------------------- | --------- |
+| Latest release / `main` | ✅        |
+| Older tagged releases   | ❌        |
+
+Deployments should track the latest release (or the `edge` image) to stay on a
+supported version.
+
+## Scope
+
+**In scope**
+
+- The application code in this repository (the SvelteKit app, its server
+  endpoints, and the published Docker image).
+
+**Out of scope** (please report elsewhere or expect a lower priority)
+
+- The Yivi / IRMA infrastructure itself — report those to the
+  [Yivi project](https://github.com/privacybydesign).
+- Vulnerabilities in third-party dependencies with no demonstrated impact here;
+  report upstream, and we will pick up the fix via a dependency bump.
+- Demo / seed data and example values (e.g. the `*.example.nl` fixtures).
+- Findings that require a compromised device, physical access, or social
+  engineering, and volumetric denial-of-service.