POST /reload can return null instead of GeoJSON

**File:** web/app.py:257

`POST /reload` returns `JSONResponse(cache.get_current())` without checking for `None`.

**Failure scenario:** On a fresh deployment where Valkey has never been populated and FIRMS returns zero detections, `reload_data()`'s empty-features branch never calls `cache.set_current()`, so `get_current()` is still `None`. The route returns HTTP 200 with a literal `null` body instead of a GeoJSON FeatureCollection, breaking any caller (including a scheduled `mise run reload`) expecting GeoJSON.

**Category:** correctness

Start at web/app.py:257 and trace POST /reload through reload_data()'s empty-features branch and the cache calls. Verify the fresh-deployment case where FIRMS returns zero detections, and ensure the endpoint returns a GeoJSON FeatureCollection rather than null.

Written by the indexing model from the issue text.