feat: serve built frontend from backend with SPA fallback

backend/app/main.py

@@ -1,11 +1,16 @@
 from contextlib import asynccontextmanager
+from pathlib import Path
 
 from fastapi import FastAPI
+from fastapi.responses import FileResponse
+from fastapi.staticfiles import StaticFiles
 
 import app.models  # noqa: F401 — registers models on Base.metadata before create_all
 from app.db import Base, engine
 from app.routes.auth import router as auth_router
 
+FRONTEND_DIST = Path(__file__).resolve().parent.parent.parent / "frontend" / "dist"
+
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
@@ -21,3 +26,11 @@ app.include_router(auth_router)
 @app.get("/healthz")
 async def healthz():
     return {"status": "ok"}
+
+
+app.mount("/assets", StaticFiles(directory=FRONTEND_DIST / "assets"), name="frontend-assets")
+
+
+@app.get("/{full_path:path}")
+async def serve_spa(full_path: str):
+    return FileResponse(FRONTEND_DIST / "index.html")

backend/tests/test_frontend_serving.py

@@ -0,0 +1,16 @@
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_unknown_path_serves_spa_index(client):
+    response = await client.get("/some/client/side/route")
+    assert response.status_code == 200
+    assert '<div id="root">' in response.text
+
+
+@pytest.mark.asyncio
+async def test_auth_routes_still_work_alongside_spa_fallback(client):
+    response = await client.post(
+        "/auth/register", json={"username": "frontendcheck", "password": "spookyspooky"}
+    )
+    assert response.status_code == 201