[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/integration/services/test_project_autonomy_update.py

@@ -61,6 +61,7 @@ async def test_update_sets_autonomy_opt_ins(db_session: AsyncSession) -> None:
             ci_watch_workflow="ci.yml",
             dep_update_command="uv lock --upgrade",
             dep_update_paths=["uv.lock"],
+            video_engine_enabled=True,
         ),
     )
 
@@ -70,3 +71,4 @@ async def test_update_sets_autonomy_opt_ins(db_session: AsyncSession) -> None:
     assert reloaded.ci_watch_workflow == "ci.yml"
     assert reloaded.dep_update_command == "uv lock --upgrade"
     assert reloaded.dep_update_paths == ["uv.lock"]
+    assert reloaded.video_engine_enabled is True