feat(search): let each user pick their own default book languages (#1255)

## Why

`BOOK_LANGUAGE` is a per-reader property, not a per-instance one. On a
shared install one household member searches in German while another
wants English and German — today whoever changes the setting changes it
for everyone, and the only escape is re-picking languages in the filter
on every single search.

The per-user override machinery already carries `SEARCH_MODE`, the
metadata providers and the default release sources, so the language
default mostly had to opt into it.

## What changed

**The field.** `BOOK_LANGUAGE` becomes `user_overridable` and moves from
the **General** tab to **Search Mode**, next to the other
user-overridable search defaults (per
[review](https://github.com/calibrain/shelfmark/pull/1255#issuecomment-5391189094)
— the first version had the Search section span two tabs, this one
doesn't). Admins set it per user in the user editor, users set it in
**My Account → Search Preferences**, and the Search Mode tab carries the
usual "N users override this" summary.

**No migration for the move.** `general` and `search_mode` both persist
into `settings.json`, and a field's value is resolved through
`load_config_file(tab)` for the tab it's declared on — so an install
that already stores `BOOK_LANGUAGE` keeps its value. Checked against a
`settings.json` written while the field still lived on General: the
stored value resolves unchanged, a fresh install still gets `["en"]`,
and `BOOK_LANGUAGE` in the environment still overrides both.

**The two places the default is read.**

- `/api/config` seeds the frontend's language filter, so it now resolves
`BOOK_LANGUAGE` for the session user.
- `build_release_search_plan` falls back to the default whenever a
request carries no language filter — which is exactly what the filter's
"Default" option sends. It takes an optional `user_id`, passed by
`/api/releases` from the session and by the Prowlarr retry path from
`task.user_id`, so a retry re-searches in the languages of whoever
queued the download.

**Validation.** Overrides go through `normalize_language()`, so
`"German"`, `"ger"` and `"de"` all store as `de`, and an unknown
language is rejected with a message naming it instead of being silently
searched for. An empty list stays an empty list (a deliberate "no
default filter"), `null` clears the override as everywhere else, and ENV
still wins: with `BOOK_LANGUAGE` set in the environment the field
reports `fromEnv` and overrides are ignored.

**Scope.** Only the language default becomes overridable. The two format
lists left behind under "Default Search Filters" stay admin-only — they
describe what the library and its post-processing accept, not what a
reader wants to read. There's a test pinning that.

## Verification

- 2681 unit tests pass (2670 before, 11 added)
- `ruff check`, `ruff format`, `basedpyright` over backend and tests,
and `vulture` all clean; frontend lint, format, typecheck and 126 unit
tests clean
- `docs/environment-variables.md` regenerated via
`scripts/generate_env_docs.py` (the `BOOK_LANGUAGE` row follows the
field into the Search Mode section)
- Manually against a two-user instance with builtin auth (first round,
before the tab move): with user A on German and user B on
English+German, `/api/config` returns each reader their own
`default_language` and an unfiltered `/api/releases` plans the matching
languages; an admin can set and read the same override for another user;
clearing it falls back to the global value; a stray `"klingon"` is
rejected; and `BOOK_LANGUAGE` in the environment overrides both users
with the field marked `fromEnv`
- After the tab move I re-ran the suites above plus the
stored-value/fresh-install/ENV check described under "No migration for
the move"; the behaviour it exercises is what the move could have broken

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: CaliBrain <calibrain@l4n.xyz>

docs/environment-variables.md

@@ -298,15 +298,6 @@ Audiobook formats to include in search results. ZIP/RAR archives are extracted a
 - **Type:** string (comma-separated)
 - **Default:** `m4b,mp3,m4a,mp4,flac,ogg,wma,aac,wav,opus,zip,rar`
 
-#### `BOOK_LANGUAGE`
-
-**Default Book Languages**
-
-Default language filter for searches.
-
-- **Type:** string (comma-separated)
-- **Default:** `en`
-
 </details>
 
 ## Search Mode
@@ -314,6 +305,7 @@ Default language filter for searches.
 | Variable | Description | Type | Default |
 |----------|-------------|------|---------|
 | `SEARCH_MODE` | How you want to search for and download books. | string (choice) | `universal` |
+| `BOOK_LANGUAGE` | Default language filter for searches. Users can override this for their own account. | string (comma-separated) | `en` |
 | `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` |
@@ -337,6 +329,15 @@ How you want to search for and download books.
 - **Default:** `universal`
 - **Options:** `direct` (Direct), `universal` (Universal)
 
+#### `BOOK_LANGUAGE`
+
+**Default Book Languages**
+
+Default language filter for searches. Users can override this for their own account.
+
+- **Type:** string (comma-separated)
+- **Default:** `en`
+
 #### `AA_DEFAULT_SORT`
 
 **Default Sort Order**
@@ -749,6 +750,7 @@ Automatically open the downloads sidebar when a new download is queued.
 Automatically download completed files to your browser for the selected content types.
 
 - **Type:** string (comma-separated)
+  
 - **Default:** _empty list_
 
 #### `MAX_CONCURRENT_DOWNLOADS`

docs/users-and-requests.md

@@ -30,7 +30,7 @@ Requires mounting your Calibre-Web `app.db` to `/auth/app.db`.
 
 Admins can configure per-user settings by editing a user in the user management panel. Non-admin users can also edit their own settings through **My Account** (accessible from the user menu). Admins control which sections are visible in My Account via the **Visible Self-Settings Sections** option.
 
-There are three categories of per-user settings:
+There are four categories of per-user settings:
 
 ### Delivery Preferences
 
@@ -42,6 +42,15 @@ Override where a user's downloads are sent. Options depend on the global output
 - **BookLore library/path** — Per-user BookLore target (when using BookLore output mode)
 - **Email recipient** — Per-user email address (when using Email output mode)
 
+### Search Preferences
+
+Override how a user searches, on top of the global search defaults:
+
+- **Search mode** — Direct or Universal for this user
+- **Default book languages** — The languages a user's searches fall back to when they don't pick one themselves. Useful for a shared instance where readers want different languages.
+- **Metadata providers** — Book, audiobook, and combined-mode provider for this user
+- **Default release sources** — The release tab opened first for books and audiobooks
+
 ### Notifications
 
 Users can configure personal notification routes, separate from the global notification settings. Each route targets a URL (e.g. an Apprise-compatible endpoint) and can be scoped to specific event types or all events.