docs(analytics,jobs): Work Order 3 batch 4 — device usage, popular books, job create/status

DeviceUsageResponse / PopularBooksResponse / services.JobResult pinned by
type; jobs create uses an inline anonymous request struct (RawMessage pin).
Two UNVERIFIED markers recorded (analytics date-range params, facet value
element shape).

docs/developer/api/analytics/get_device_usage.md

@@ -0,0 +1,40 @@
+---
+method: GET
+path: /api/analytics/device-usage
+auth: jwt
+area: analytics
+---
+<!-- source: internal/handlers/analytics.go DeviceUsageResponse / GetDeviceUsage -->
+
+# Get Device Usage
+
+Per-device sync activity for the signed-in user.
+
+## Example response
+
+```json example-response
+{
+  "devices": [
+    {
+      "device_id": "550e8400-e29b-41d4-a716-446655440000",
+      "device_name": "Kobo Clara",
+      "device_type": "kobo",
+      "sync_count": 42,
+      "last_sync": "2026-10-24T13:57:01Z",
+      "total_time_seconds": 3600,
+      "total_time_minutes": 60
+    }
+  ]
+}
+```
+
+## Query parameters
+
+UNVERIFIED: accepts the standard analytics date-range params; confirm exact
+names before relying on them.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 500 | `{"error": "..."}` | unexpected server error |