24x7 RTMP desktop stream, client side + UAT PASS (#798)

Whole-screen KDE capture -> x264 -> RTMP, fenced to the rt slice.
Loopback mediamtx receiver; Cloudron cutover = one creds edit.
Detail: https://projects.knownelement.com/issues/798#note-4519

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Assisted-by: Crush:glm-5.2

rtmp/docker-compose.yml

@@ -0,0 +1,13 @@
+# Local RTMP receiver for the 24x7 desktop stream (#798).
+# Loopback-only ingest (127.0.0.1:1935) used for UAT and as a local mirror;
+# the real target is the human's Cloudron streaming server (URL lands in
+# ~/.creds/rtmp-stream.env when provided — service restart applies it).
+name: ultix-rtmp
+services:
+  rtmp-rx:
+    image: bluenviron/mediamtx:1.20.1
+    container_name: ukrrs-ultix-rtmp-rx
+    network_mode: host
+    volumes:
+      - ./mediamtx.yml:/mediamtx.yml:ro
+    restart: unless-stopped