Address review findings on target_fps validation and config resolution

- Reject floats and bools before int() in the target_fps save path. A JSON
  body can carry them, where int(90.5) silently stored 90 and true stored 1.
  Form posts send strings, so '90.5' already failed in int().
- Assert the template's target_fps is 100, not merely an int, so the
  documented default is actually pinned.
- Empty-config precedence: keeping the `and config` check deliberately, now
  spelled out in the comment and covered by a test. Both managers default to
  the same config/config.json, so falling through cannot pick up a different
  file's settings; treating {} as an answer would instead return {} when the
  first manager simply hasn't loaded yet, silently disabling every setting
  read through the property -- the failure this property exists to fix.

Suite 1446 passed. The float-rejection test was checked to fail without the
guard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/blueprints/api_v3.py

@@ -753,8 +753,18 @@ def save_main_config():
         # would have been quietly altered is reported rather than appearing to
         # save and then behaving differently.
         if 'target_fps' in data and data['target_fps'] not in ('', None):
+            raw_target_fps = data['target_fps']
+            # A JSON body can carry real floats and bools, where int() would
+            # silently truncate: 90.5 would save as 90, and true as 1. Reject
+            # them rather than storing a value the user did not ask for. Form
+            # posts arrive as strings, so '90.5' still fails in int() below.
+            if isinstance(raw_target_fps, (bool, float)):
+                return jsonify({
+                    'status': 'error',
+                    'message': "Invalid value for target_fps: must be an integer"
+                }), 400
             try:
-                target_fps = int(data['target_fps'])
+                target_fps = int(raw_target_fps)
             except (ValueError, TypeError):
                 return jsonify({
                     'status': 'error',