Add content_type=combined URL parameter and FORCE_COMBINED_SEARCH user setting (#1058)

Adds support for `content_type=combined` in URL search parameters,
letting users force combined-mode searches via a bookmarkable link
rather than relying on the last-used preference from localStorage.

The override is applied only in Universal mode and only when combined
mode is actually available (universal enabled, `show_combined_selector`
on, neither content type blocked by policy). Otherwise, it's silently
ignored, consistent with how `content_type=ebook`/`audiobook` already
behave outside Universal.

Existing `content_type=ebook`/`audiobook` URLs now also force combined
mode off, so the URL is authoritative regardless of prior preference.

Also adds a per-user `FORCE_COMBINED_SEARCH` setting that locks combined
mode on whenever it's available.

URL `content_type=ebook`/`audiobook` overrides are also superseded by
force-combined for the same reason: the search bar wouldn't let users
switch back, so honoring the URL param would leave them in a state they
couldn't escape from.

tests/core/test_admin_users_api.py

@@ -1267,6 +1267,7 @@ class TestAdminSearchPreferences:
         assert data["keys"] == [
             "SEARCH_MODE",
             "SHOW_COMBINED_SELECTOR",
+            "FORCE_COMBINED_SEARCH",
             "METADATA_PROVIDER",
             "METADATA_PROVIDER_AUDIOBOOK",
             "METADATA_PROVIDER_COMBINED",