synth-diagnostic starts pipeline services, cleans up on failure

- run-tests.sh: starts pipeline services, runs test-unit.py,
  stops services on failure, leaves running on success
- synth-diagnostic is the single gate for pipeline startup
- deploy: stop pipeline → restart prometheus → start synth-diagnostic
  (synth-diagnostic starts pipeline on success)

justfile

@@ -104,9 +104,9 @@ deploy:
         cd {{REPO_DIR}} && git pull && \
         chmod +x scripts/*.py scripts/*.sh 2>/dev/null; \
         echo '{{BOARD_PASS}}' | sudo -S systemctl daemon-reload && \
+        echo '{{BOARD_PASS}}' | sudo -S systemctl stop midi-bridge pd-synth-onboard led-matrix-viz && \
         echo '{{BOARD_PASS}}' | sudo -S systemctl restart prometheus-exporter && \
         echo '{{BOARD_PASS}}' | sudo -S systemctl start synth-diagnostic && \
-        echo '{{BOARD_PASS}}' | sudo -S systemctl restart midi-bridge pd-synth-onboard led-matrix-viz && \
         echo '[OK] Deployed + restarted'"
 
 # Start synth (onboard audio)