Add SDR device registry to prevent decoder conflicts

Implements centralized tracking of SDR device allocation to prevent
multiple decoders from trying to use the same device simultaneously.

- Add sdr_device_registry with claim/release/status functions in app.py
- Update all SDR-based routes to claim devices on start and release on stop
- Return HTTP 409 with DEVICE_BUSY error when device is already in use
- Clear registry on /killall
- Skip device claims for remote connections (rtl_tcp, remote SBS)

Fixes #100
Fixes #101

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app.py

@@ -27,7 +27,7 @@ from typing import Any
 
 from flask import Flask, render_template, jsonify, send_file, Response, request,redirect, url_for, flash, session
 from werkzeug.security import check_password_hash
-from config import VERSION, CHANGELOG, SHARED_OBSERVER_LOCATION_ENABLED
+from config import VERSION, CHANGELOG, SHARED_OBSERVER_LOCATION_ENABLED
 from utils.dependencies import check_tool, check_all_dependencies, TOOL_DEPENDENCIES
 from utils.process import cleanup_stale_processes
 from utils.sdr import SDRFactory
@@ -216,6 +216,52 @@ cleanup_manager.register(adsb_aircraft)
 cleanup_manager.register(ais_vessels)
 cleanup_manager.register(dsc_messages)
 
+# ============================================
+# SDR DEVICE REGISTRY
+# ============================================
+# Tracks which mode is using which SDR device to prevent conflicts
+# Key: device_index (int), Value: mode_name (str)
+sdr_device_registry: dict[int, str] = {}
+sdr_device_registry_lock = threading.Lock()
+
+
+def claim_sdr_device(device_index: int, mode_name: str) -> str | None:
+    """Claim an SDR device for a mode.
+
+    Args:
+        device_index: The SDR device index to claim
+        mode_name: Name of the mode claiming the device (e.g., 'sensor', 'rtlamr')
+
+    Returns:
+        Error message if device is in use, None if successfully claimed
+    """
+    with sdr_device_registry_lock:
+        if device_index in sdr_device_registry:
+            in_use_by = sdr_device_registry[device_index]
+            return f'SDR device {device_index} is in use by {in_use_by}. Stop {in_use_by} first or use a different device.'
+        sdr_device_registry[device_index] = mode_name
+        return None
+
+
+def release_sdr_device(device_index: int) -> None:
+    """Release an SDR device from the registry.
+
+    Args:
+        device_index: The SDR device index to release
+    """
+    with sdr_device_registry_lock:
+        sdr_device_registry.pop(device_index, None)
+
+
+def get_sdr_device_status() -> dict[int, str]:
+    """Get current SDR device allocations.
+
+    Returns:
+        Dictionary mapping device indices to mode names
+    """
+    with sdr_device_registry_lock:
+        return dict(sdr_device_registry)
+
 
 # ============================================
 # MAIN ROUTES
@@ -271,22 +317,22 @@ def login():
     return render_template('login.html', version=VERSION)
 
 @app.route('/')
-def index() -> str:
-    tools = {
-        'rtl_fm': check_tool('rtl_fm'),
-        'multimon': check_tool('multimon-ng'),
-        'rtl_433': check_tool('rtl_433'),
-        'rtlamr': check_tool('rtlamr')
-    }
-    devices = [d.to_dict() for d in SDRFactory.detect_devices()]
-    return render_template(
-        'index.html',
-        tools=tools,
-        devices=devices,
-        version=VERSION,
-        changelog=CHANGELOG,
-        shared_observer_location=SHARED_OBSERVER_LOCATION_ENABLED,
-    )
+def index() -> str:
+    tools = {
+        'rtl_fm': check_tool('rtl_fm'),
+        'multimon': check_tool('multimon-ng'),
+        'rtl_433': check_tool('rtl_433'),
+        'rtlamr': check_tool('rtlamr')
+    }
+    devices = [d.to_dict() for d in SDRFactory.detect_devices()]
+    return render_template(
+        'index.html',
+        tools=tools,
+        devices=devices,
+        version=VERSION,
+        changelog=CHANGELOG,
+        shared_observer_location=SHARED_OBSERVER_LOCATION_ENABLED,
+    )
 
 
 @app.route('/favicon.svg')
@@ -629,6 +675,10 @@ def kill_all() -> Response:
         dsc_process = None
         dsc_rtl_process = None
 
+    # Clear SDR device registry
+    with sdr_device_registry_lock:
+        sdr_device_registry.clear()
+
     return jsonify({'status': 'killed', 'processes': killed})