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_popular_books.md

@@ -0,0 +1,39 @@
+---
+method: GET
+path: /api/analytics/popular-books
+auth: jwt
+area: analytics
+---
+<!-- source: internal/handlers/analytics.go PopularBooksResponse / GetPopularBooks -->
+
+# Get Popular Books
+
+Most-read books for the signed-in user.
+
+## Example response
+
+```json example-response
+{
+  "books": [
+    {
+      "media_item_id": "550e8400-e29b-41d4-a716-446655440000",
+      "title": "1984",
+      "author": "George Orwell",
+      "read_count": 3,
+      "avg_completion": 0.95,
+      "last_read": "2026-10-24T13:57:01Z"
+    }
+  ]
+}
+```
+
+## 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 |