Broadcaster preview player pointed at /test mount

[anneoneone]

Goal

Give the broadcaster a preview player pointed at the /test mount so they can hear themselves before going live (Phase 2 of #164, req #2).

Tasks

• Broadcaster preview player pointed at /test (plain <audio src>), visible only to the broadcaster.

Files

Admin SPA (frontend/src/admin/...) — add the preview to the broadcast flow / StreamPage.vue.

Acceptance (Phase 2)

Broadcaster can push to /test, hear themselves via the preview player, and curl https://<host>/status-json.xsl shows the listeners count move — all without touching /live.

Depends on

#164 Phase-2 stack ticket (the /test mount) and the producer/Icecast-output ticket.

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Parent: #164 (Phase 2).