add prometheus exporter + test suite

- prometheus-exporter.py: stdlib-only HTTP metrics on :9090/metrics
- Exposes: ALSA status, hw_ptr rate, service health, mixer controls
- MIDI counters: notes_on, notes_off, errors
- test-unit.py: 10-test suite covering full MIDI→Pd→Audio pipeline
- test-tone.pd, test-midi-log.pd: minimal debug patches
- systemd service for exporter
- install.sh/deploy updated to include exporter

pd/test-midi-log.pd

@@ -0,0 +1,7 @@
+#N canvas 50 50 400 300 10;
+#X obj 50 70 netreceive -u 8081;
+#X obj 50 110 route midi;
+#X obj 50 150 print MIDI;
+#X text 50 30 TEST: log MIDI received on UDP 8081;
+#X connect 0 0 1 0;
+#X connect 1 0 2 0;

pd/test-tone.pd

@@ -0,0 +1,8 @@
+#N canvas 50 50 400 300 10;
+#X obj 50 50 osc~ 440;
+#X obj 50 90 *~ 0.3;
+#X obj 50 130 dac~;
+#X text 50 30 TEST: 440Hz tone direct to DAC (no MIDI);
+#X connect 0 0 1 0;
+#X connect 1 0 2 0;
+#X connect 1 0 2 1;