feat(scenarios): implement execution engine contracts

backend/src/app.py

@@ -3,8 +3,11 @@
 # @BRIEF The main entry point for the FastAPI application.
 # @LAYER API
 # @RELATION DEPENDS_ON -> [Api.Init.ApiRoutesModule]
+# @RELATION CALLS -> [Dependencies.AppDependencies.LiveExecutionComposition]
 # @INVARIANT All WebSocket connections must be properly cleaned up on disconnect.
 # @INVARIANT All WebSocket connections must be authenticated via JWT or API key token (see [SEC:C-3]).
+# @INVARIANT Startup initializes a fail-closed 044 LiveExecutionCompositionRoot; runtime providers
+#            must be registered server-side and never inferred from HTTP/run metadata.
 # @PRE Python environment and dependencies installed; configuration database available.
 # @POST FastAPI app instance is created, middleware configured, and routes registered.
 # @SIDE_EFFECT Starts background scheduler and binds network ports for HTTP/WS traffic.
@@ -86,19 +89,47 @@ from .core.logger import belief_scope, logger
 from .core.utils.network import NetworkError
 from .dependencies import (
     get_async_job_runner,
+    get_config_manager,
     get_current_user,
+    get_live_execution_composition_root,
     get_scheduler_service,
     get_task_manager,
 )
 from .models.auth import Role, User
 
 
+# #region App.AppModule.LiveExecutionCompositionBootstrap [C:4] [TYPE Function] [SEMANTICS scenario,execution,composition,startup]
+# @BRIEF Register trusted deployment live bindings before the scheduler can dispatch queued ScenarioRuns.
+# @RELATION CALLS -> [ScenarioExecution.LiveCompositionRoot.Bootstrap]
+# @POST Default dispatcher sees exact configured 037 providers or typed unavailable bindings; HTTP/run
+#       metadata never creates a live client.
+# @INVARIANT Browser and Screenshot providers are separately registered server-side; their absence does
+#            not prevent startup and remains typed unavailable.
+# @REJECTED Deferring Superset provider creation to a queued run was rejected — that would derive
+#            execution authority from a persisted/request identifier instead of deployment config.
+def initialize_live_execution_composition() -> int:
+    from .services.dashboard_testing.execution.live_composition import (
+        bootstrap_live_execution_composition,
+    )
+
+    return bootstrap_live_execution_composition(
+        get_live_execution_composition_root(),
+        config_manager=get_config_manager(),
+        run_async=get_async_job_runner().run,
+    )
+# #endregion App.AppModule.LiveExecutionCompositionBootstrap
+
+
 # #region App.AppModule.Lifespan [C:3] [TYPE Function]
 # @ingroup Module
 # @BRIEF Async context manager for FastAPI startup/shutdown lifecycle.
 # @RELATION CALLS -> [Core.Database.InitDb]
 # @RELATION CALLS -> [Dependencies.AppDependencies]
+# @RELATION CALLS -> [App.AppModule.LiveExecutionCompositionBootstrap]
 # @POST On startup: admin exists, scheduler started. On shutdown: scheduler stopped.
+# @INVARIANT The trusted live-composition bootstrap runs after AsyncJobRunner initialization and
+#            before scheduler startup; queued dispatch therefore cannot observe an unbootstrapped
+#            default root during normal application startup.
 # @RATIONALE Alembic migrations removed from lifespan — they now run exclusively
 #   in docker/backend.entrypoint.sh (wait_for_db → alembic upgrade head).
 #   Running migrations in both places added ~5s startup overhead and masked
@@ -193,6 +224,9 @@ async def lifespan(app: FastAPI):
 
         logger.reason("Initializing AsyncJobRunner")
         get_async_job_runner()  # Initialize singleton with running event loop BEFORE scheduler starts
+        logger.reason("Bootstrapping fail-closed ScenarioRun live composition root")
+        live_bindings = initialize_live_execution_composition()
+        logger.reason("ScenarioRun live composition bootstrap complete", payload={"bindings": live_bindings})
         logger.reason("Starting scheduler")
         scheduler = get_scheduler_service()
     scheduler.start()