add boot-time synth-diagnostic.service

- synth-diagnostic.service: runs test-unit.py before pipeline services
- Type=oneshot, After=prometheus-exporter, Before=pipeline services
- Pipeline services: After=synth-diagnostic
- deploy: restart prometheus-exporter → start synth-diagnostic → restart pipeline
- install.sh: includes synth-diagnostic in SERVICES array

justfile

@@ -104,7 +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 restart midi-bridge pd-synth-onboard led-matrix-viz prometheus-exporter && \
+        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)