app: SPA fallback for arbitrary client routes

StaticFiles(html=True) only serves index.html for directory paths,
which 404s on /scenarios, /what-if, /scenarios/123 — anything React
Router owns. Subclass StaticFiles to catch the 404 from get_response
and return index.html so the SPA can take over routing client-side.

API routes still match first (under /api/* in prod), so no risk of
shadowing.

Found via headless verification through chrome-service: dashboard
loaded 200 + nav rendered, but /scenarios + /what-if returned 404.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

fire_planner/app.py

@@ -33,9 +33,12 @@ from pathlib import Path
 from typing import Any
 
 from fastapi import Depends, FastAPI, status
+from fastapi.exceptions import HTTPException
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
 from prometheus_fastapi_instrumentator import Instrumentator
+from starlette.types import Scope
 
 from fire_planner.api.auth import require_bearer
 from fire_planner.api.goals import router as goals_router
@@ -144,11 +147,29 @@ async def healthz() -> dict[str, Any]:
     return {"status": "ok", "queue_depth": depth}
 
 
-# Mount the SPA last so it catches any path the API didn't claim. With
-# `html=True`, requests for paths like `/scenarios` (no file at that
-# path) return `index.html`, letting React Router own client routing.
+class _SPAStaticFiles(StaticFiles):
+    """StaticFiles with a SPA fallback: any 404 returns index.html so
+    React Router can own client-side routing for paths like /scenarios,
+    /what-if, /scenarios/123, etc.
+
+    `StaticFiles(html=True)` only serves index.html for *directories*,
+    not arbitrary not-found paths — that's not enough for a SPA.
+    """
+
+    async def get_response(self, path: str, scope: Scope):  # type: ignore[no-untyped-def]
+        try:
+            return await super().get_response(path, scope)
+        except HTTPException as exc:
+            if exc.status_code == 404:
+                index = Path(self.directory) / "index.html"  # type: ignore[arg-type]
+                if index.is_file():
+                    return FileResponse(index)
+            raise
+
+
+# Mount the SPA last so it catches any path the API didn't claim.
 if _FRONTEND_DIST and Path(_FRONTEND_DIST).is_dir():
-    app.mount("/", StaticFiles(directory=_FRONTEND_DIST, html=True), name="spa")
+    app.mount("/", _SPAStaticFiles(directory=_FRONTEND_DIST, html=True), name="spa")
     log.info("Mounted SPA from %s; API at %s/*", _FRONTEND_DIST, _API_PREFIX)
 elif _FRONTEND_DIST:
     log.warning("FRONTEND_DIST=%s does not exist; SPA not mounted", _FRONTEND_DIST)