feat: add runnable simulator foundation

app/main.py

@@ -0,0 +1,34 @@
+"""FastAPI application factory and ASGI entry point."""
+
+from __future__ import annotations
+
+from fastapi import FastAPI
+
+from app.api.errors import unhandled_exception_handler
+from app.api.middleware import RequestContextMiddleware
+from app.config import Settings, get_settings
+from app.lifespan import DatabaseFactory, create_lifespan, default_database_factory
+from app.logging import configure_logging
+from app.observability.health import router as health_router
+
+
+def create_app(
+    settings: Settings | None = None,
+    database_factory: DatabaseFactory = default_database_factory,
+) -> FastAPI:
+    """Create an isolated application instance with explicit resource factories."""
+
+    resolved = settings or get_settings()
+    configure_logging(resolved.log_level)
+    app = FastAPI(
+        title=resolved.app_name,
+        version="0.0.1",
+        lifespan=create_lifespan(resolved, database_factory),
+    )
+    app.add_middleware(RequestContextMiddleware, header_name=resolved.request_id_header)
+    app.add_exception_handler(Exception, unhandled_exception_handler)
+    app.include_router(health_router)
+    return app
+
+
+app = create_app()