fix: stop bt-a2dp-jack during profile changes to prevent JACK race

The bt-a2dp-jack systemd unit runs an independent JACK instance for
Bluetooth audio. When stop_jack() kills jackd, bt-a2dp-jack restarts
it within 5 seconds with default parameters. By the time start_jack()
runs in the profile handler, _jack_is_running() already returns True
for the bt-a2dp JACK, causing start_jack() to return immediately
without configuring our period/rate. The pedal's JACK client then
fails with server_error (0x21) because the running JACK doesn't match.

Fix:
- stop_jack() temporarily stops bt-a2dp-jack before killing jackd
- Profile handler restarts bt-a2dp-jack after JACK is configured
- Rollback path also restarts bt-a2dp-jack

src/system/audio.py

@@ -371,6 +371,15 @@ class AudioSystem:
 
     def stop_jack(self) -> None:
         """Gracefully stop the JACK server."""
+        # Stop the Bluetooth A2DP bridge first — it restarts jackd
+        # independently and can race with our profile changes.
+        try:
+            subprocess.run(
+                ["systemctl", "stop", "bt-a2dp-jack"],
+                capture_output=True, timeout=5,
+            )
+        except Exception:
+            pass
         try:
             subprocess.run(
                 ["killall", "jackd"],

src/web/server.py

@@ -1739,6 +1739,12 @@ class WebServer:
                         jack_client.start()
                     except Exception:
                         pass
+                # Restart bt-a2dp bridge even on rollback so Bluetooth audio recovers
+                try:
+                    import subprocess
+                    subprocess.run(["systemctl", "start", "bt-a2dp-jack"], capture_output=True, timeout=5)
+                except Exception:
+                    pass
                 raise HTTPException(
                     status_code=500,
                     detail=f"Failed to start JACK with period={target_profile['period']}, rate={target_profile['rate']} — rolled back",
@@ -1787,6 +1793,16 @@ class WebServer:
                 except Exception as e:
                     logger.warning("connect_fx_ports() failed after profile switch: %s", e)
 
+            # Restart Bluetooth A2DP bridge so it can connect to our JACK
+            try:
+                import subprocess
+                subprocess.run(
+                    ["systemctl", "start", "bt-a2dp-jack"],
+                    capture_output=True, timeout=5,
+                )
+            except Exception:
+                pass
+
             # ── Persist to config.yaml ──────────────────────────────
             if self.deps.config is not None:
                 audio_cfg = self.deps.config.setdefault("audio", {})