[TASK] Add security policy

SECURITY.md

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+# Security Policy
+
+This product ("web-vision/deepl-write") is developed and maintained by web-vision GmbH.
+As a manufacturer of a product with digital elements under the EU Cyber
+Resilience Act (Regulation (EU) 2024/2847), we are committed to identifying,
+assessing, and remediating security vulnerabilities in this product in a
+timely manner, and to coordinating responsibly with security researchers.
+
+## Supported Versions
+
+Security updates are provided for the following versions. Versions marked
+unsupported no longer receive security fixes; please upgrade before
+reporting an issue against them.
+
+| Version        | Supported           |
+| -------------- | ------------------- |
+| 2.x            | :white_check_mark:  |
+| 1.x            | :white_check_mark:  |
+| < 1.0          | :x:                 |
+
+Planned end of support for this product: **30 June 2029 (end of regular TYPO3 14 LTS support)**.
+
+## Reporting a Vulnerability
+
+Please report suspected security vulnerabilities privately — **do not**
+open a public GitHub/GitLab issue.
+
+- **Report here:** https://security.web-vision.de (our secure incident report form)
+- **Please include:** affected version(s), a description of the issue,
+  steps to reproduce or a proof of concept, and the potential impact.
+
+### What to expect
+
+| Step                         | Timeframe                         |
+| ----------------------------- | ---------------------------------- |
+| Acknowledgement of your report | within 1 business day (typically much faster) |
+| Status updates                | at least every 7 days until resolved |
+| Fix / mitigation, based on severity | Critical/High: as fast as possible; Medium/Low: next scheduled release |
+
+We coordinate the disclosure timeline with the reporter and aim for a
+resolution before any public disclosure. If you'd like credit for your
+finding, let us know how you'd like to be named; we're also happy to keep
+your report confidential if you prefer.
+
+## Safe Harbor
+
+We consider security research conducted in good faith, in accordance with
+this policy, to be authorized. We will not pursue legal action against
+researchers who:
+
+- make a genuine effort to avoid privacy violations, data destruction, and
+  service interruption during their research,
+- report a vulnerability promptly and do not exploit it beyond what is
+  necessary to demonstrate the issue,
+- do not publicly disclose the vulnerability before we have had a
+  reasonable opportunity to address it (see timeframes above).
+
+## Scope
+
+In scope: the source code, released versions, and official distribution
+channels of "web-vision/deepl-write" (e.g. TER / Packagist).
+
+Out of scope: third-party dependencies (please report those upstream, but
+feel free to let us know so we can track and update them), and
+vulnerabilities in the host application (TYPO3 core) itself
+unless directly caused by this extension.
+
+## Coordinator
+
+Vulnerability reports for this product are handled by the
+Vulnerability Coordinator team at web-vision GmbH,
+under the oversight of our GRC/CISO function.