fix: Settings silently not saved — sl_trust_mode, enable_isbn_lookup, duplicate field

[deucebucket]

Bug

Three settings save issues found:

1. `sl_trust_mode` silently not saved — `settings.html:459` has the select dropdown, but the POST handler in app.py never reads `request.form.get('sl_trust_mode')`. Changes are silently discarded. The setting IS used at `layer_api.py:88`.

2. `enable_isbn_lookup` silently not saved — `settings.html:343` has the checkbox, but it's never read by the POST handler. Used at `app.py:2232`.

3. Duplicate `google_books_api_key` field — `settings.html:829` (Engine tab) and `settings.html:959` (Pipeline tab) both have `name="google_books_api_key"`. On POST, the browser sends the last value, silently overwriting edits made on the Engine tab.

Fix

1. Add `config['sl_trust_mode'] = request.form.get('sl_trust_mode', 'full')` to the settings POST handler
2. Add `config['enable_isbn_lookup'] = request.form.get('enable_isbn_lookup') == 'on'` to the POST handler
3. Remove the duplicate field at settings.html:959 (or whichever is the redundant one)

Severity

High — settings appear to save but changes are silently lost.

Found via UI flow audit.