Resolve 29 open CodeQL security alerts across 5 files

py/flask-debug (#214):
- debug_web_manual.py: read debug mode from LEDMATRIX_FLASK_DEBUG env var
  instead of hardcoded True

py/stack-trace-exposure (#216, #218):
- api_v3.py execute_system_action: remove subprocess stdout/stderr from
  HTTP responses; log via logger instead
- api_v3.py get_git_version: validate output matches safe ref format
  (^[a-zA-Z0-9._-]+$) before including in response
- api_v3.py: remove all remaining traceback.format_exc() dead variables
  and print() debug calls (replaced with logger.debug/warning)

py/reflective-xss (#207, #208, #209, #210, #211, #212):
- api_v3.py: remove plugin_id from all error/success response messages
  (uninstall, install, update, health, not-found responses)
- pages_v3.py load_partial: return static "Partial not found" message
  instead of echoing partial_name
- pages_v3.py _load_starlark_config_partial: add app_id regex validation,
  use static error messages instead of f-strings with app_id

py/path-injection (#187–#206):
- pages_v3.py _load_plugin_config_partial: resolve plugins_base and
  validate _plugin_dir with relative_to() before all file operations;
  same for assets metadata directory
- pages_v3.py _load_starlark_config_partial: resolve starlark_base and
  validate schema_file/config_file paths with relative_to()
- plugin_loader.py _find_plugin_directory: resolve plugins_dir and
  validate strategy-2 candidates with relative_to()
- plugin_loader.py install_dependencies: resolve plugin_dir first, then
  construct requirements_file and marker_path from resolved base
- plugin_loader.py load_module: resolve plugin_dir with strict=True and
  validate entry_file with relative_to() before exec_module

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

scripts/debug/debug_web_manual.py

@@ -67,8 +67,9 @@ def main():
         print("   📍 Will run on: http://0.0.0.0:5000")
         print("   ⏹️  Press Ctrl+C to stop")
 
-        # Run the app (this should start the server)
-        app.run(host='0.0.0.0', port=5000, debug=True)
+        # Run the app (debug mode controlled by env var to satisfy security scanners)
+        _debug = os.environ.get('LEDMATRIX_FLASK_DEBUG', '0') == '1'
+        app.run(host='0.0.0.0', port=5000, debug=_debug)
 
     except KeyboardInterrupt:
         print("\n   ⏹️  Server stopped by user")