Closes#151. The middleware was already implemented + tested (3 passing
tests in securityheaders_test.go covering base set, multi-route, and
the don't-override-existing contract) but never registered in router.go.
One-line wire-up, runs after TenantGuard so rejected requests still
get the same headers as accepted ones, and before routes so handlers
can still opt out by setting their own header before c.Next() returns.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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