fix: SSTV VIS detector stuck in DETECTED state on validation failure

The previous fix (f29ae3d) introduced a regression: when VIS parity
check failed or the VIS code was unrecognized, the detector entered
DETECTED state permanently and never resumed scanning. Now it resets
to IDLE on validation failure and only enters DETECTED on success.

Also resets partial image progress counter between consecutive decodes
and adds SDR device claiming to general SSTV route to prevent conflicts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/sstv_general.py

@@ -13,6 +13,7 @@ from pathlib import Path
 
 from flask import Blueprint, Response, jsonify, request, send_file
 
+import app as app_module
 from utils.logging import get_logger
 from utils.sse import format_sse
 from utils.event_pipeline import process_event
@@ -27,6 +28,9 @@ sstv_general_bp = Blueprint('sstv_general', __name__, url_prefix='/sstv-general'
 # Queue for SSE progress streaming
 _sstv_general_queue: queue.Queue = queue.Queue(maxsize=100)
 
+# Track which device is being used
+_sstv_general_active_device: int | None = None
+
 # Predefined SSTV frequencies
 SSTV_FREQUENCIES = [
     {'band': '80 m', 'frequency': 3.845, 'modulation': 'lsb', 'notes': 'Common US SSTV calling frequency', 'type': 'Terrestrial HF'},
@@ -150,6 +154,17 @@ def start_decoder():
             'message': 'Modulation must be fm, usb, or lsb',
         }), 400
 
+    # Claim SDR device
+    global _sstv_general_active_device
+    device_int = int(device_index)
+    error = app_module.claim_sdr_device(device_int, 'sstv_general')
+    if error:
+        return jsonify({
+            'status': 'error',
+            'error_type': 'DEVICE_BUSY',
+            'message': error,
+        }), 409
+
     # Set callback and start
     decoder.set_callback(_progress_callback)
     success = decoder.start(
@@ -159,6 +174,7 @@ def start_decoder():
     )
 
     if success:
+        _sstv_general_active_device = device_int
         return jsonify({
             'status': 'started',
             'frequency': frequency,
@@ -166,6 +182,7 @@ def start_decoder():
             'device': device_index,
         })
     else:
+        app_module.release_sdr_device(device_int)
         return jsonify({
             'status': 'error',
             'message': 'Failed to start decoder',
@@ -175,8 +192,14 @@ def start_decoder():
 @sstv_general_bp.route('/stop', methods=['POST'])
 def stop_decoder():
     """Stop general SSTV decoder."""
+    global _sstv_general_active_device
     decoder = get_general_sstv_decoder()
     decoder.stop()
+
+    if _sstv_general_active_device is not None:
+        app_module.release_sdr_device(_sstv_general_active_device)
+        _sstv_general_active_device = None
+
     return jsonify({'status': 'stopped'})