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/link_book.md

@@ -0,0 +1,59 @@
+---
+method: POST
+path: /api/devices/:deviceId/sync/link-book
+auth: jwt
+area: devices
+---
+<!-- source: internal/handlers/matching.go:57 MatchingHandler.LinkBook -->
+
+# Link Book
+
+Links a device file (by path + SHA256) to a media item, optionally with a
+confidence score. Used by the book-matching flow.
+
+## Example request
+
+```json example-request
+{
+  "device_file": {
+    "file_path": "/books/1984.epub",
+    "sha256": "abc123...",
+    "title": "1984"
+  },
+  "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+  "confidence_score": 0.95
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "status": "linked",
+  "device_file_alias": {
+    "id": "660e8400-e29b-41d4-a716-446655440000",
+    "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+    "device_id": "770e8400-e29b-41d4-a716-446655440000",
+    "file_path": "/books/1984.epub",
+    "file_sha256": "abc123...",
+    "confidence_score": 0.95
+  }
+}
+```
+
+## Request fields
+
+| Field | Type | Required | Notes |
+| ----- | ---- | -------- | ----- |
+| device_file.file_path | string | no | path on the device |
+| device_file.sha256 | string | no | file content hash |
+| device_file.title | string | no | device-side title |
+| media_item_id | string (uuid) | yes | `services.LinkBookRequest` declares it as uuid; the gate only checks the Bind struct — UNVERIFIED: confirm required tag on LinkBookRequest fields |
+| confidence_score | number | no | match confidence (0-1) |
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "Invalid device ID"}` / `{"error": "Invalid request body"}` | malformed id or body |
+| 500 | `{"error": "Failed to link book: ..."}` | service failure |