Fix SPA serving: use HTMLResponse instead of FileResponse

backend/main.py

@@ -1,8 +1,10 @@
 from contextlib import asynccontextmanager
 
+import os
+
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
-from fastapi.responses import FileResponse
+from fastapi.responses import HTMLResponse
 
 from backend.database import engine
 from backend.routers import health, subreddits, posts, comments, authors, analytics, digests, summaries
@@ -28,12 +30,15 @@ app.include_router(summaries.router, prefix="/api/v1")
 
 
 # SPA static file serving (only when frontend is built)
-import os
+STATIC_DIR = "/app/static"
+_index_html = ""
 
-static_dir = os.path.join(os.path.dirname(__file__), "..", "static")
-if os.path.isdir(static_dir):
-    app.mount("/assets", StaticFiles(directory=os.path.join(static_dir, "assets")), name="assets")
+if os.path.isdir(STATIC_DIR):
+    with open(os.path.join(STATIC_DIR, "index.html")) as f:
+        _index_html = f.read()
+
+    app.mount("/assets", StaticFiles(directory=os.path.join(STATIC_DIR, "assets")), name="assets")
 
     @app.get("/{full_path:path}")
     async def serve_spa(full_path: str):
-        return FileResponse(os.path.join(static_dir, "index.html"))
+        return HTMLResponse(_index_html)