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>

tests/core/test_admin_users_api.py

@@ -508,6 +508,32 @@ class TestAdminUserUpdateEndpoint:
         settings = user_db.get_user_settings(user["id"])
         assert settings["DESTINATION_AUDIOBOOK"] == "/audiobooks/alice"
 
+    def test_update_user_settings_normalizes_book_languages(self, admin_client, user_db):
+        user = user_db.create_user(username="alice")
+
+        resp = admin_client.put(
+            f"/api/admin/users/{user['id']}",
+            json={"settings": {"BOOK_LANGUAGE": ["German", "de", " en "]}},
+        )
+        assert resp.status_code == 200
+        settings = user_db.get_user_settings(user["id"])
+        assert settings["BOOK_LANGUAGE"] == ["de", "en"]
+
+    def test_update_user_settings_rejects_unknown_book_language(self, admin_client, user_db):
+        user = user_db.create_user(username="alice")
+
+        resp = admin_client.put(
+            f"/api/admin/users/{user['id']}",
+            json={"settings": {"BOOK_LANGUAGE": ["de", "klingon"]}},
+        )
+        assert resp.status_code == 400
+        assert resp.json["error"] == "Invalid settings payload"
+        assert any(
+            "BOOK_LANGUAGE contains an unsupported language: klingon" in msg
+            for msg in resp.json["details"]
+        )
+        assert user_db.get_user_settings(user["id"]) == {}
+
     def test_update_user_settings_accepts_notification_overrides(self, admin_client, user_db):
         user = user_db.create_user(username="alice")
 
@@ -1266,6 +1292,7 @@ class TestAdminSearchPreferences:
         assert data["tab"] == "search_mode"
         assert data["keys"] == [
             "SEARCH_MODE",
+            "BOOK_LANGUAGE",
             "SHOW_COMBINED_SELECTOR",
             "FORCE_COMBINED_SEARCH",
             "METADATA_PROVIDER",
@@ -1295,6 +1322,21 @@ class TestAdminSearchPreferences:
         assert data["effective"]["DEFAULT_RELEASE_SOURCE_AUDIOBOOK"]["source"] == "user_override"
         assert data["effective"]["DEFAULT_RELEASE_SOURCE_AUDIOBOOK"]["value"] == "audiobookbay"
 
+    def test_reports_book_language_override(self, admin_client, user_db):
+        user = user_db.create_user(username="alice")
+        user_db.set_user_settings(user["id"], {"BOOK_LANGUAGE": ["de", "en"]})
+
+        resp = admin_client.get(f"/api/admin/users/{user['id']}/search-preferences")
+        assert resp.status_code == 200
+
+        data = resp.json
+        assert data["userOverrides"]["BOOK_LANGUAGE"] == ["de", "en"]
+        assert data["effective"]["BOOK_LANGUAGE"] == {
+            "value": ["de", "en"],
+            "source": "user_override",
+        }
+        assert data["globalValues"]["BOOK_LANGUAGE"] == ["en"]
+
     def test_returns_404_for_unknown_user(self, admin_client):
         resp = admin_client.get("/api/admin/users/9999/search-preferences")
         assert resp.status_code == 404