docs(devices): Work Order 3 batch 1 — 6 missing device endpoints + gate entries

New frontmatter'd docs with handler-compatible examples:
- devices/get_device.md (DeviceInfo shape)
- devices/regenerate_device_token.md (token + sync_urls)
- devices/get_device_mappings.md (wrapper + DeviceMappingSummary)
- devices/update_device_mapping.md (UpdateDeviceMappingRequest)
- devices/download_sidecar_config.md (sidecar config JSON file)
- devices/link_book.md (LinkBookRequest; one UNVERIFIED marker on required-tag)

Schema registry: 6 new entries; DeviceMappingSummary hoisted to package
scope. Gate passes (the UNVERIFIED marker is logged, not a failure).

Excludes /api/devices-page (htmx page route, per ground rules).

docs/developer/api/devices/download_sidecar_config.md

@@ -0,0 +1,42 @@
+---
+method: GET
+path: /api/devices/:id/sidecar/download
+auth: jwt
+area: devices
+---
+<!-- source: internal/handlers/sidecar.go:215 SidecarHandler.DownloadSidecarConfig -->
+
+# Download Sidecar Config
+
+Generates and downloads a `.bookhoard.json` sidecar configuration file for a
+device (used by the sidecar sync workflow). The body is a JSON document returned
+with `Content-Disposition: attachment`.
+
+## Example response
+
+```json example-response
+{
+  "version": "1.0",
+  "bookhoard": {
+    "opds_catalog": "http://localhost:8765/opds/devices/550e8400-.../catalog",
+    "sync_api": "http://localhost:8765/api/sync/kobo",
+    "opds_base_url": "http://localhost:8765/opds",
+    "api_base_url": "http://localhost:8765/api",
+    "device_id": "550e8400-e29b-41d4-a716-446655440000",
+    "device_token": "dev_abc123..."
+  },
+  "books": {},
+  "collections": [],
+  "opds_enabled": true,
+  "sidecar_enabled": true,
+  "last_updated": "2026-10-24T13:57:01Z"
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid device ID"}` | malformed id |
+| 404 | `{"error": "device not found"}` | no such device |
+| 500 | `{"error": "failed to fetch media items"}` / `{"error": "failed to generate config"}` | unexpected server error |