fix: extension updates install the new build — no 12h-cached "latest" XPI, and the popup's Update opens FC's install page

Operator, 2026-09-25: "the extension update trigger from inside the
extension doesn't work and the manual update seems to not move it to the
most recent version or at least mark it the most recent."

- fabledcurator-latest.xpi was served with Quart's default
  `public, max-age=43200`: one URL whose bytes change every release, so a
  browser that had fetched it reinstalled the previous build for 12 hours
  (measured on the instance). It is now `no-cache` (the ETag keeps an
  unchanged file a 304); versioned XPIs are `immutable`.
- The web Settings card installs/downloads the VERSIONED xpi_url, which can
  only ever be that build's bytes.
- The popup's Update button did tabs.create() on the .xpi, which Firefox
  refuses (NS_ERROR_FAILURE on a 200: it only installs from a user click on
  a web page). It now opens FC's install card (/subscriptions?tab=settings).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_api_extension.py

@@ -779,6 +779,28 @@ async def test_serve_extension_latest_returns_most_recent_xpi(
     assert data == b"new"
 
 
+@pytest.mark.asyncio
+async def test_the_latest_alias_is_never_served_from_a_stale_cache(
+    client, monkeypatch, tmp_path,
+):
+    """One URL whose bytes change every release: a cached copy reinstalls the
+    previous build (operator-flagged 2026-09-25, when it was max-age=43200)."""
+    (tmp_path / "fabledcurator-1.0.1.xpi").write_bytes(b"new")
+    monkeypatch.setattr(frontend_module, "XPI_DIR", tmp_path)
+    resp = await client.get("/extension/fabledcurator-latest.xpi")
+    assert resp.headers["Cache-Control"] == "no-cache"
+    assert "Expires" not in resp.headers
+
+
+@pytest.mark.asyncio
+async def test_a_versioned_xpi_is_cached_for_good(client, monkeypatch, tmp_path):
+    """A versioned name is one build's bytes forever."""
+    (tmp_path / "fabledcurator-1.0.1.xpi").write_bytes(b"new")
+    monkeypatch.setattr(frontend_module, "XPI_DIR", tmp_path)
+    resp = await client.get("/extension/fabledcurator-1.0.1.xpi")
+    assert "immutable" in resp.headers["Cache-Control"]
+
+
 @pytest.mark.asyncio
 async def test_serve_extension_latest_404_when_dir_empty(client, monkeypatch, tmp_path):
     monkeypatch.setattr(frontend_module, "XPI_DIR", tmp_path)