fix(routes): 500 (not 404) when the app-shell index.html is missing (#4791)

Follow-up to #4637. serve_index — the handler for / and the SPA deep-link
routes (/notes, /calendar, /cookbook, /email, /memory, /gallery, /tasks,
/library) — pre-checked os.path.exists and raised its own
HTTPException(404, "index.html not found") when the bundle was missing. So a
missing core template returned 404 before serve_html_with_nonce's 500 could
fire, the one inconsistency left after #4637.

index.html is a fixed, app-bundled template; a missing one is a broken
deployment (server fault), not a client "not found", so it should surface as a
logged 500 in 5xx alerting rather than a 404. Keep the static->root fallback,
drop the redundant existence guard and the dead-end 404, and let the shared
helper handle the missing case.

Verified against the running app: / and /notes return 200 with the bundle
present and a logged 500 when index.html is absent.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -806,10 +806,12 @@ async def serve_index(request: Request):
     static_path = abs_join(BASE_DIR, "static/index.html")
     if os.path.exists(static_path):
         return serve_html_with_nonce(request, static_path)
-    root_path = abs_join(BASE_DIR, "index.html")
-    if os.path.exists(root_path):
-        return serve_html_with_nonce(request, root_path)
-    raise HTTPException(404, "index.html not found")
+    # No static bundle — fall back to a root-level index.html if one is shipped.
+    # If neither exists, serve_html_with_nonce logs it and returns a generic 500:
+    # a missing index.html is a broken deployment (server fault), not a client
+    # "not found". This keeps the app-shell route consistent with the other
+    # bundled-template routes instead of mislabelling the fault as a 404.
+    return serve_html_with_nonce(request, abs_join(BASE_DIR, "index.html"))
 
 @app.get("/notes")
 async def serve_notes(request: Request):