feat(slice-4): backend serves the built SPA — the deployed nginx-proxies-all topology (launch-app §2)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/main.py

@@ -17,6 +17,7 @@ from typing import Any
 import psycopg
 from fastapi import Depends, FastAPI, Response
 from fastapi.responses import JSONResponse
+from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel
 
 from app.domains import accounts, storefronts
@@ -89,7 +90,7 @@ def _set_session_cookie(response: Response, account: accounts.Account) -> None:
     )
 
 
-def create_app(database_url: str | None = None) -> FastAPI:
+def create_app(database_url: str | None = None, static_dir: str | Path | None = None) -> FastAPI:
     _ensure_app_logging()
     dsn = database_url or config.database_url()
 
@@ -202,6 +203,13 @@ def create_app(database_url: str | None = None) -> FastAPI:
             )
         return JSONResponse(status_code=201, content={"id": sf.id, "name": sf.name})
 
+    # Deployed topology (launch-app SPEC §2): nginx proxies everything here, so the
+    # backend serves the built SPA. Mounted LAST so /healthz and /api/* win. In dev the
+    # dist dir doesn't exist (Vite serves the frontend) and the mount is skipped.
+    spa_dir = Path(static_dir) if static_dir is not None else _REPO_ROOT / "frontend" / "dist"
+    if (spa_dir / "index.html").is_file():
+        app.mount("/", StaticFiles(directory=spa_dir, html=True), name="spa")
+
     return app