synth-diagnostic: just run tests, no service management

- run-tests.sh: no systemctl start/stop, just runs test-unit.py
- synth-diagnostic.service: removed Before=, keeps After=prometheus-exporter
  (no deadlock risk since run-tests.sh doesn't start services)
- justfile deploy: stop pipeline -> restart prometheus -> start
  synth-diagnostic -> restart pipeline

Pipeline starts independently at boot (no After=synth-diagnostic).
Diagnostic provides visibility at boot without blocking startup.

system/synth-diagnostic.service

@@ -2,7 +2,6 @@
 Description=Synth pipeline diagnostic — runs unit tests at boot
 DefaultDependencies=no
 After=multi-user.target prometheus-exporter.service
-Before=midi-bridge.service pd-synth-onboard.service led-matrix-viz.service
 
 [Service]
 Type=oneshot
@@ -10,9 +9,6 @@ ExecStart=/home/arduino/uno-q-audio-synth/scripts/run-tests.sh
 StandardOutput=journal
 StandardError=journal
 RemainAfterExit=yes
-
-# Don't let failures cascade — critical for headless boot
-# On failure: services won't start, board stays SSH-accessible
 FailureAction=none
 
 [Install]