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/jobs/get_job_status.md

@@ -0,0 +1,34 @@
+---
+method: GET
+path: /api/jobs/:jobId
+auth: jwt
+area: jobs
+---
+<!-- source: internal/handlers/jobs.go:76 GetJobStatus; services/worker.go JobResult -->
+
+# Get Job Status
+
+Returns the in-memory status/result of a background job.
+
+## Example response
+
+```json example-response
+{
+  "job_id": "ee0e8400-e29b-41d4-a716-446655440000",
+  "status": "completed",
+  "error": "",
+  "result": null,
+  "progress": 100,
+  "files_scanned": 120,
+  "new_items": 3,
+  "errors": 0
+}
+```
+
+`library_id` and `job_type` appear only on scan jobs (`omitempty`).
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 404 | `{"error": "Job not found"}` | unknown id, or the in-memory result was evicted by a restart |