fix: prevent app crash when frontend build is missing

StaticFiles defaults to check_dir=True, which raises at import time if
frontend/dist/assets is missing on restart — taking down /healthz and
/auth/* along with the frontend. Pass check_dir=False so the mount never
crashes the app, and make the SPA fallback return a clear 503 instead of
an unhandled 500 when index.html is absent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/main.py

@@ -1,7 +1,7 @@
 from contextlib import asynccontextmanager
 from pathlib import Path
 
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
 
@@ -28,9 +28,19 @@ async def healthz():
     return {"status": "ok"}
 
 
-app.mount("/assets", StaticFiles(directory=FRONTEND_DIST / "assets"), name="frontend-assets")
+app.mount(
+    "/assets",
+    StaticFiles(directory=FRONTEND_DIST / "assets", check_dir=False),
+    name="frontend-assets",
+)
 
 
 @app.get("/{full_path:path}")
 async def serve_spa(full_path: str):
-    return FileResponse(FRONTEND_DIST / "index.html")
+    index_file = FRONTEND_DIST / "index.html"
+    if not index_file.exists():
+        raise HTTPException(
+            status_code=503,
+            detail="Frontend not built. Run `npm run build` in frontend/ and restart.",
+        )
+    return FileResponse(index_file)