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/regenerate_device_token.md

@@ -0,0 +1,56 @@
+---
+method: PUT
+path: /api/devices/:id/regenerate-token
+auth: jwt
+area: devices
+---
+<!-- source: internal/handlers/devices.go:443 DeviceHandler.RegenerateDeviceToken -->
+
+# Regenerate Device Token
+
+Mints a fresh device auth token and returns it together with device-specific
+sync URLs. The previous token is invalidated.
+
+For Kobo devices the response includes `sync_urls` (init/markup/bookmark/sync_url);
+for KOReader devices it contains the KOReader endpoints.
+
+## Example response
+
+```json example-response
+{
+  "message": "Token regenerated successfully",
+  "auth_token": "dev_abc123...",
+  "device": {
+    "id": "550e8400-e29b-41d4-a716-446655440000",
+    "device_name": "Kobo Clara",
+    "device_type": "kobo",
+    "last_sync": null,
+    "last_seen": null,
+    "sync_enabled": true,
+    "auto_sync": true,
+    "sync_frequency_minutes": 60,
+    "created_at": "2026-10-24T13:57:01Z",
+    "device_metadata": null
+  },
+  "sync_urls": {
+    "sync_url": "http://localhost:8765/api/sync/kobo/dev_abc123...",
+    "markup": "http://localhost:8765/api/sync/kobo/dev_abc123.../markup",
+    "bookmark": "http://localhost:8765/api/sync/kobo/dev_abc123.../bookmark",
+    "init": "http://localhost:8765/api/sync/kobo/dev_abc123.../v1/initialization"
+  }
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid device ID"}` | malformed id |
+| 401 | `{"error": "invalid user ID"}` | no/invalid JWT |
+| 403 | `{"error": "access denied"}` | device belongs to another user |
+| 404 | `{"error": "device not found"}` | no such device |
+| 500 | `{"error": "..."}` | token generation / DB failure |
+
+Notes: HTMX requests get an HTML reload snippet instead of JSON.
+`sync_urls` keys differ by device type (Kobo gets the four above; KOReader
+gets its own set).