backing tracks: add source selector + bluealsa in device list

- Audio devices endpoint now includes Bluetooth Audio (bluealsa) and
  other software PCM devices from aplay -L
- Added GET/POST /api/backing-source endpoints with JACK port routing
- Backing Track Source dropdown in Audio section (None, Bluetooth
  Audio, or any JACK capture port found)
- Selecting a source routes it into fx_in:input_2 via jack_connect
- Also supports bt_audio:capture_1/2 from existing JACK bridge

src/web/server.py

@@ -2110,10 +2110,79 @@ iface eth0 inet static
                     if "USB" in line or "usb" in line.lower():
                         m = re.match(r"\s*(\d+)", line)
                         if m: usb_id = int(m.group(1))
+                # Add software PCM devices (bluealsa, etc.) as virtual cards
+                try:
+                    r3 = subprocess.run(["aplay","-L"], capture_output=True, text=True, timeout=3)
+                    for line in r3.stdout.splitlines():
+                        name = line.strip()
+                        if name.startswith("bluealsa") or name.startswith("bluealsa:"):
+                            if not any(c.get("name","") == "Bluetooth Audio" for c in cards):
+                                cards.append({"card": -1, "name": "Bluetooth Audio",
+                                              "device": 0, "desc": "BlueALSA (BT playback)"})
+                except: pass
                 return {"devices": cards, "usb_card": usb_id, "suggested": f"hw:{usb_id or 0},0"}
             except Exception as e:
                 return {"error": str(e), "devices": []}
 
+        # ── Backing track source ────────────────────────────────────
+
+        @app.get("/api/backing-source")
+        async def get_backing_source():
+            """Get current backing track source and available sources."""
+            import subprocess
+            audio = self.deps.audio_system
+            cfg = audio.config.__dict__ if audio and hasattr(audio.config, '__dict__') else {}
+            current = cfg.get("backing_source", "none")
+            # Check available JACK ports for possible sources
+            sources = [{"id": "none", "name": "None"}]
+            try:
+                r = subprocess.run(["jack_lsp"], capture_output=True, text=True, timeout=3)
+                for line in r.stdout.splitlines():
+                    line = line.strip()
+                    if line.endswith(":capture_1") or line.endswith(":capture_2"):
+                        name = line.split(":")[0]
+                        if name != "system" and name != "pi-multifx" and name != "bt_audio":
+                            sources.append({"id": name, "name": f"{name} (capture)"})
+                # Always offer bluetooth if JACK has bt_audio ports
+                r2 = subprocess.run(["jack_lsp","bt_audio:capture_1"], capture_output=True, text=True, timeout=2)
+                if r2.returncode == 0:
+                    sources.append({"id": "bt_audio", "name": "Bluetooth Audio"})
+            except: pass
+            return {"current": current, "sources": sources}
+
+        @app.post("/api/backing-source")
+        async def set_backing_source(data: dict):
+            """Set backing track source and route it into the FX chain.
+
+            Body: { "source": "none" | "bt_audio" | "<jack_source>" }
+            """
+            import subprocess
+            source = data.get("source", "none")
+            audio = self.deps.audio_system
+            # Save to config
+            if audio and hasattr(audio.config, '__dict__'):
+                audio.config.__dict__["backing_source"] = source
+            # Disconnect old backing connections
+            try:
+                r = subprocess.run(["jack_lsp","fx_in:input_2"], capture_output=True, text=True, timeout=2)
+                if r.returncode == 0:
+                    subprocess.run(["jack_disconnect","bt_audio:capture_1","fx_in:input_2"],
+                                   capture_output=True, timeout=2)
+                    subprocess.run(["jack_disconnect","bt_audio:capture_2","fx_in:input_2"],
+                                   capture_output=True, timeout=2)
+            except: pass
+            # Connect new source
+            if source != "none":
+                try:
+                    if source == "bt_audio":
+                        subprocess.run(["jack_connect","bt_audio:capture_1","fx_in:input_2"],
+                                       capture_output=True, timeout=3)
+                    else:
+                        subprocess.run(["jack_connect",f"{source}:capture_1","fx_in:input_2"],
+                                       capture_output=True, timeout=3)
+                except: pass
+            return {"ok": True, "source": source}
+
         @app.get("/api/levels")
         async def get_signal_levels():
             levels = {"channels": {}}