docs(api): document unified tunable system settings endpoints

The scattered scan-settings JSON routes are superseded by the new
admin-only /api/system/settings pair backed by the SettingsRegistry
(introduced in 885f6d8 / bc47450).

- Rewrite system/settings.md around GET/PUT /api/system/settings:
  SettingEntry metadata shape (type, min/max, requires_restart,
  category, group, is_default), type-aware validation rules, and the
  full tunable-setting catalog (scanner, general, security, api, sync,
  performance) with defaults, ranges, and restart requirements
- Note the legacy /api/libraries/scan-settings routes as back-compat
  only (they now refresh the registry cache on write)
- Add system/config.md for GET/PUT /api/system/config: raw key/value
  system configuration (e.g. base_url), including validation notes and
  guidance to prefer the typed settings endpoint for registry keys

docs/developer/api/system/config.md

@@ -0,0 +1,77 @@
+# System Config API
+
+## Overview
+
+Raw key/value system configuration storage (backed by the `system_config` table). Unlike the typed [System Settings API](settings.md), this endpoint reads and writes arbitrary config keys as plain strings — including keys without registry metadata, such as `base_url`.
+
+**Base URL**: `/api/system`
+**Authentication**: Admin JWT token required
+**Content-Type**: `application/json`
+
+---
+
+## Endpoints
+
+### Get System Configuration
+
+Retrieve all system configuration entries as a flat key/value map.
+
+**Endpoint**: `GET /api/system/config`
+
+**Authentication**: Admin role required
+
+**Response**: **200 OK**
+
+```json
+{
+  "base_url": "http://192.168.1.100:8765",
+  "default_timezone": "America/New_York"
+}
+```
+
+**Example**:
+
+```bash
+curl -X GET https://bookhoard.example.com/api/system/config \
+  -H "Authorization: Bearer <admin_token>"
+```
+
+---
+
+### Update System Configuration
+
+Update one or more config values.
+
+**Endpoint**: `PUT /api/system/config`
+
+**Authentication**: Admin role required
+
+**Request Body**: a flat map of keys to string values. Only the supplied keys are updated.
+
+```json
+{
+  "base_url": "https://bookhoard.example.com"
+}
+```
+
+**Validation**: values for known keys are validated where applicable — for example, `default_timezone` must be a valid IANA timezone (`time.LoadLocation`); invalid values return `400` without persisting.
+
+**Response**: **200 OK** on success; `400` (invalid value/format), `401`, `403`, `500` on failure.
+
+**Example**:
+
+```bash
+curl -X PUT https://bookhoard.example.com/api/system/config \
+  -H "Authorization: Bearer <admin_token>" \
+  -H "Content-Type: application/json" \
+  -d '{"base_url": "https://bookhoard.example.com"}'
+```
+
+> **Note:** settings that appear in the typed settings registry (e.g. `default_timezone`) are better managed through [`PUT /api/system/settings`](settings.md), which also returns metadata and reload hints. Writes through either endpoint refresh the shared registry cache.
+
+---
+
+## Related Endpoints
+
+- [System Settings API](settings.md) — typed, validated tunable settings with metadata
+- `GET /api/devices/:id/sidecar` — device setup config derived from system config (see [Devices API](../devices/))