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/create_job.md

@@ -0,0 +1,40 @@
+---
+method: POST
+path: /api/jobs
+auth: jwt
+area: jobs
+---
+<!-- source: internal/handlers/jobs.go CreateJob -->
+
+# Create Job
+
+Enqueues a background job (scan, import, convert — the set accepted by the
+worker). Success is **202 Accepted** with the job id; poll its status via
+`GET /api/jobs/:jobId`.
+
+## Example request
+
+```json example-request
+{
+  "type": "import",
+  "params": { "library_id": "990e8400-e29b-41d4-a716-446655440000" }
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "message": "Job created",
+  "job_id": "ee0e8400-e29b-41d4-a716-446655440000",
+  "type": "import",
+  "status": "pending"
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "Invalid job type"}` / others | unknown type or params |
+| 500 | `{"error": "..."}` | enqueue failure |