Foreground Web Motion Detection Execution Prompt

Implement automatic foreground-only motion detection for the web camera role using the artifacts below as the source of truth:

Delivery Rules

Treat the implementation plan as authoritative for architecture and sequencing.
Complete checklist items atomically and update the validation matrix with evidence as work lands.
Do not silently skip hidden work such as store updates, detector lifecycle cleanup, tests, docs, and runtime validation.
Reuse the existing backend motion lifecycle before introducing any new backend motion pipeline.
Preserve manual motion controls and existing client notification behavior.

Every completed checklist item must have at least one evidence entry in the validation matrix.
Code evidence must reference the file changed.
Test evidence must reference a deterministic automated or manual validation step.
Runtime evidence must reference an observed app behavior, command result, or manual scenario outcome.

Automatic motion detection works only for the web camera role.
The detector runs only while the browser tab is in the foreground and the detector is armed.
The detector starts and ends existing motion events automatically without duplicate starts.
Client notifications and recordings continue to function through existing backend routes.
Manual motion controls still work.
Detector settings are user-visible and persisted locally.
Tests and runtime validation cover idle scenes, real motion, flicker, and camera shake.