fix(sim): revalidate /media instead of pinning it immutable

The `immutable, max-age=1y` media header pinned stale footage in the browser for
a year, so a re-sourced or re-cropped clip (e.g. the de-captioned cosmos) never
showed without a manual cache clear. Switch /media to `no-cache` (store, but
revalidate every load): instant altitude swaps already come from the in-memory
blob preload, so the HTTP cache only matters on (re)load — where a conditional
request is a cheap 304 when unchanged but picks up edited clips immediately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_simulator_api.py

@@ -113,8 +113,11 @@ def test_media_is_cacheable(tmp_path, monkeypatch):
     resp = client.get("/media/cosmos/base.mp4")
     assert resp.status_code == 200
     cc = resp.headers.get("cache-control", "")
+    # Stored but revalidated: not no-store (so the blob preload can cache it), and
+    # not pinned immutable (so a re-sourced/re-cropped clip shows on a plain reload).
     assert "no-store" not in cc
-    assert "max-age" in cc
+    assert "immutable" not in cc
+    assert "no-cache" in cc
 
 
 @pytest.fixture