all: annotate gosec false positives with rationale

Each //nolint:gosec carries the gosec code and one line on why
the finding is a false positive or already mitigated.

  G124 cookies (oidc.go x3, oidc_confirm_test.go)
    Secure is set conditionally on req.TLS != nil; HttpOnly and
    SameSiteStrictMode already on. gosec misses the conditional.
    Test fixture cookie is explicitly a test fixture.

  G705 (debug.go)
    templates.PingPage(...).Render() is a templ component that
    auto-escapes user input.

  G706 (scenario.go)
    Integration log emits trusted scenario state. The pre-built
    image G706 sites in hsic.go / tsic.go ride along with the
    earlier constants commit.

  G710 (app.go, tailsql.go)
    Redirect target is "trusted ServerURL prefix + path". gosec
    cannot see past the prefix.