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.

scripts/run-tests.sh

@@ -1,25 +1,18 @@
 #!/bin/bash
-# run-tests.sh — start services, run tests, clean up on failure
+# run-tests.sh — run synth pipeline diagnostics
 set -euo pipefail
 
 BOARD="${BOARD_HOST:-192.168.9.167}"
 cd /home/arduino/uno-q-audio-synth
 
-echo "[diag] Starting pipeline services for diagnostics..."
-sudo systemctl start midi-bridge pd-synth-onboard led-matrix-viz
-
-echo "[diag] Waiting 3s for services to settle..."
-sleep 3
-
 echo "[diag] Running synth pipeline diagnostics against $BOARD..."
 python3 scripts/test-unit.py "$BOARD"
 exit_code=$?
 
 if [ $exit_code -eq 0 ]; then
-    echo "[diag] All tests PASSED — pipeline healthy, leaving services running"
+    echo "[diag] All tests PASSED"
 else
-    echo "[diag] Tests FAILED (exit $exit_code) — stopping pipeline services"
-    sudo systemctl stop midi-bridge pd-synth-onboard led-matrix-viz
+    echo "[diag] Tests FAILED (exit $exit_code)"
 fi
 
 exit $exit_code