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URL `content_type=ebook`/`audiobook` overrides are also superseded by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.

docs/environment-variables.md

@@ -223,6 +223,7 @@ Default language filter for searches.
 | `AA_DEFAULT_SORT` | Default sort order for search results. | string (choice) | `relevance` |
 | `SHOW_RELEASE_SOURCE_LINKS` | Show clickable release-source links in release and details modals. Metadata provider links stay enabled. | boolean | `true` |
 | `SHOW_COMBINED_SELECTOR` | Show the option to search for and download both a book and audiobook together. | boolean | `true` |
+| `FORCE_COMBINED_SEARCH` | Force combined search whenever it's available. Locks the combined toggle on. | boolean | `false` |
 | `METADATA_PROVIDER` | Choose which metadata provider to use for book searches. | string (choice) | `openlibrary` |
 | `METADATA_PROVIDER_AUDIOBOOK` | Metadata provider for audiobook searches. Uses the book provider if not set. | string (choice) | _empty string_ |
 | `METADATA_PROVIDER_COMBINED` | Metadata provider for combined mode searches. Uses the book provider if not set. | string (choice) | _empty string_ |
@@ -270,6 +271,15 @@ Show the option to search for and download both a book and audiobook together.
 - **Type:** boolean
 - **Default:** `true`
 
+#### `FORCE_COMBINED_SEARCH`
+
+**Always Use Combined Search**
+
+Force combined search whenever it's available. Locks the combined toggle on.
+
+- **Type:** boolean
+- **Default:** `false`
+
 #### `METADATA_PROVIDER`
 
 **Book Metadata Provider**