Add /api/audio/profile endpoints for runtime latency profile switching

- GET /api/audio/profiles — list available profiles with params
- GET /api/audio/profile — current profile + jack_running + xrun_count
- POST /api/audio/profile {profile: 'standard'|'low'} — switch profile,
  restart JACK server + JACK audio client atomically
- Adds audio_system + jack_audio refs to WebServerDeps
- Rollback on JACK restart failure

src/web/server.py

@@ -41,6 +41,7 @@ from ..system.tonedownload import (
     discover_anon_key,
     format_size,
 )
+from ..system.audio import AudioSystem, JackAudioClient, LATENCY_PROFILES
 
 logger = logging.getLogger(__name__)
 
@@ -201,6 +202,11 @@ class WebServerDeps:
     bass_nam_host: Optional[NAMHost] = None
     bass_ir_loader: Optional[IRLoader] = None
 
+    # Audio system (for JACK restart on profile change)
+    audio_system: Optional[AudioSystem] = None
+    # JACK audio client (needs restart when JACK restarts)
+    jack_audio: Optional[JackAudioClient] = None
+
     @property
     def is_connected(self) -> bool:
         """True if we're wired to a live pedal instance (any channel)."""
@@ -1269,6 +1275,106 @@ class WebServer:
             from ..system.bluetooth import bt_midi_connected_devices
             return {"devices": bt_midi_connected_devices()}
 
+        # ── Audio latency profile endpoints ───────────────────────
+
+        @app.get("/api/audio/profiles")
+        async def list_audio_profiles():
+            """List all available JACK latency profiles."""
+            return {
+                "profiles": [
+                    {
+                        "key": k,
+                        **v,
+                    }
+                    for k, v in LATENCY_PROFILES.items()
+                ],
+                "current": (
+                    self.deps.audio_system.config.profile
+                    if self.deps.audio_system
+                    else "standard"
+                ),
+            }
+
+        @app.get("/api/audio/profile")
+        async def get_audio_profile():
+            """Get the current JACK latency profile and audio stats."""
+            if not self.deps.audio_system:
+                return {
+                    "profile": "standard",
+                    "period": 256,
+                    "nperiods": 2,
+                    "rate": 48000,
+                    "jack_running": False,
+                    "xrun_count": None,
+                }
+            profile_key = self.deps.audio_system.config.profile
+            profile = self.deps.audio_system.config.latency_profile
+            xruns = self.deps.audio_system.read_xrun_count()
+            return {
+                "profile": profile_key,
+                "period": profile["period"],
+                "nperiods": profile["nperiods"],
+                "rate": profile["rate"],
+                "jack_running": True,
+                "xrun_count": xruns,
+            }
+
+        @app.post("/api/audio/profile")
+        async def set_audio_profile(data: dict):
+            """Switch JACK latency profile (restarts JACK server).
+
+            Body: { "profile": "standard" | "low" }
+
+            The switch takes effect immediately by restarting JACK with the
+            new period / nperiods / priority settings.
+            """
+            profile_key = data.get("profile", "")
+            if profile_key not in LATENCY_PROFILES:
+                raise HTTPException(
+                    status_code=400,
+                    detail=f"Unknown profile: {profile_key!r}. Options: {list(LATENCY_PROFILES.keys())}",
+                )
+            audio_sys = self.deps.audio_system
+            if not audio_sys:
+                raise HTTPException(status_code=503, detail="Audio system not available")
+            old_key = audio_sys.config.profile
+            if old_key == profile_key:
+                return {"ok": True, "profile": profile_key, "restarted": False}
+            # Apply new profile
+            audio_sys.config.profile = profile_key
+            profile = audio_sys.config.latency_profile
+            logger.info("Switching audio profile: %s → %s (period=%d, nperiods=%d)",
+                        old_key, profile_key, profile["period"], profile["nperiods"])
+            # Stop JACK audio client first (holds a live JACK C client)
+            jack_client = self.deps.jack_audio
+            if jack_client:
+                jack_client.stop()
+            # Restart JACK server with new parameters
+            ok = audio_sys.restart_jack(timeout=10)
+            if not ok:
+                # Rollback on failure
+                audio_sys.config.profile = old_key
+                audio_sys.start_jack(timeout=10)
+                if jack_client:
+                    jack_client.start()
+                raise HTTPException(status_code=500, detail=f"Failed to start JACK with profile {profile_key!r} — rolled back to {old_key!r}")
+            # Restart JACK audio client
+            if jack_client:
+                jack_client.start()
+            await self._manager.broadcast({
+                "type": "audio_profile_changed",
+                "profile": profile_key,
+                "period": profile["period"],
+                "nperiods": profile["nperiods"],
+            })
+            return {
+                "ok": True,
+                "profile": profile_key,
+                "period": profile["period"],
+                "nperiods": profile["nperiods"],
+                "restarted": True,
+            }
+
         # ── FX block param schemas ───────────────────────────────
 
         @app.get("/api/block-params/{fx_type}")