[video-engine] Per-project video_engine_enabled opt-in toggle

Mirrors ci_watch_enabled (migration 048): the global
ROBOCO_VIDEO_ENGINE_ENABLED flag arms the subsystem; the new
projects.video_engine_enabled column (migration 063) opts a repo into
authoring against its motion/ dir. VideoEngine._opted_in_project no-ops
open_video_task at the single chokepoint covering all three trigger
paths (on-release, on-spotlight, CEO on-demand) until the operator
flips it in the panel edit-project dialog. Existing projects stay
opted out (server_default=false).

tests/unit/runtime/test_video_render_loop.py

@@ -124,6 +124,7 @@ async def _seed(session: AsyncSession) -> None:
                 assigned_cell=Team.BACKEND,
                 created_by=SYSTEM_UUID,
                 is_active=True,
+                video_engine_enabled=True,
             )
         )
     await session.flush()