fix(channels): drop hardcoded 0-7 limit on set-scope endpoint

The /api/channels/<idx>/scope route rejected idx>=8 with "Channel index
out of range (0-7)" even though current firmwares expose up to 40 slots
(send_device_query reports max_channels=40 in our logs). Users couldn't
set a region scope on channels like #ubot (idx 8) or #swietokrzyskie
(idx 15) — the UI showed the modal but Save returned 400.

Use device_manager._max_channels (set from send_device_query at connect)
as the upper bound, with 8 as a safe fallback if the DM is unreachable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/routes/api.py

@@ -4087,8 +4087,16 @@ def set_channel_scope_api(index):
     Body: {"region_id": int | null}.  null removes the mapping (firmware default applies).
     """
     try:
-        if index < 0 or index > 7:
-            return jsonify({'success': False, 'error': 'Channel index out of range (0-7)'}), 400
+        # Use the device-reported max so we don't reject valid slots on
+        # firmwares that expose more than 8 channels (current firmware reports
+        # max_channels=40). Fall back to 8 when the DM isn't reachable.
+        dm = _get_dm()
+        max_channels = getattr(dm, '_max_channels', 8) if dm else 8
+        if index < 0 or index >= max_channels:
+            return jsonify({
+                'success': False,
+                'error': f'Channel index out of range (0-{max_channels - 1})',
+            }), 400
 
         data = request.get_json() or {}
         if 'region_id' not in data: