app: catch starlette.HTTPException in SPA fallback

StaticFiles is a Starlette primitive — its 404 raises
starlette.exceptions.HTTPException, NOT fastapi.HTTPException
(which subclasses Starlette's). My initial except clause caught the
subclass and let the base class propagate, so /scenarios still 404'd.

Switch to except StarletteHTTPException so both the parent and any
FastAPI subclass are caught. Verified end-to-end via chrome-service
in the next deploy.

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

fire_planner/app.py

@@ -33,11 +33,11 @@ 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.exceptions import HTTPException as StarletteHTTPException
 from starlette.types import Scope
 
 from fire_planner.api.auth import require_bearer
@@ -159,7 +159,7 @@ class _SPAStaticFiles(StaticFiles):
     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:
+        except StarletteHTTPException as exc:
             if exc.status_code == 404:
                 index = Path(self.directory) / "index.html"  # type: ignore[arg-type]
                 if index.is_file():