broker: initialize drops the dataset's bearer tokens and everything they protected

Initialize is the fresh start between users. It already cleared the
images, plots, scan result and live rugnux status; it now also drops the
tokens, resets the dataset settings (name, file prefix, unit cell, ...)
to their defaults and forgets the last collection followed through
/live/events, so after it nothing of the previous dataset is readable,
with or without its token - the broker is as open and as empty as a
freshly started one.

The auth test checks that after /initialize the data-collection
statistics and the /statistics aggregate are open and no longer carry
the previous dataset's name. SECURITY.md also lists /rugnux/status
among the protected endpoints.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PXc1zD1yxHqaR1DGdCTHL4

image_pusher/LiveCollection.cpp

@@ -68,6 +68,16 @@ void LiveCollection::EndIfOpen(const std::string &error) {
     End(0, error);
 }
 
+void LiveCollection::Forget() {
+    {
+        std::unique_lock ul(m);
+        state = Snapshot{};
+        start_message = StartMessage{};
+        start_cbor.clear();
+    }
+    cv.notify_all();
+}
+
 std::optional<std::vector<uint8_t>> LiveCollection::StartCBOR() const {
     std::unique_lock ul(m);
     if (!state.started)