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/get_device_mappings.md

@@ -0,0 +1,40 @@
+---
+method: GET
+path: /api/devices/:id/collections
+auth: jwt
+area: devices
+---
+<!-- source: internal/handlers/collections.go:645 CollectionHandler.GetDeviceMappings -->
+
+# Get Device Collection Mappings
+
+Lists the shelf-to-collection mappings configured for a device.
+
+## Example response
+
+```json example-response
+{
+  "device_id": "550e8400-e29b-41d4-a716-446655440000",
+  "mappings": [
+    {
+      "id": "660e8400-e29b-41d4-a716-446655440000",
+      "collection_id": "770e8400-e29b-41d4-a716-446655440000",
+      "collection_name": "To Read",
+      "device_shelf_name": "Kobo",
+      "sync_direction": "bidirectional",
+      "created_at": "2026-10-24 13:57:01.81234 +0000 UTC"
+    }
+  ],
+  "total": 1
+}
+```
+
+`created_at` uses Go's `time.String()` format, not RFC3339. `total` is the
+length of `mappings`.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid device id"}` | malformed id |
+| 500 | `{"error": "..."}` | unexpected server error |