docs: add SECURITY.md (private vulnerability reporting)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please report vulnerabilities privately via
+[GitHub private vulnerability reporting](https://github.com/alexta69/metube/security/advisories/new)
+(Security tab → "Report a vulnerability"). Do **not** open a public issue for
+security problems.
+
+You can expect an initial response within a few days. Please include a
+reproduction and the MeTube release version (visible in the UI footer).
+
+## Supported versions
+
+MeTube is continuously released; only the **latest release** is supported.
+Update to the current Docker image before reporting.
+
+## Scope notes
+
+MeTube ships **without authentication by design** — it is intended to run on a
+trusted network or behind an authenticating reverse proxy (see the
+[wiki](https://github.com/alexta69/metube/wiki/Reverse-proxy-configurations)).
+Reports that reduce to "the UI is reachable without a login" are expected
+behavior, not vulnerabilities.