feat(activity): shared session job registry with the scan as its first kind

The output scan now runs as the first kind of an in-memory JobRegistry
(py/api/jobs.py, replacing py/api/scan_job.py). Each job keeps exact
counters, at most 200 outcome rows per group and a log capped at 2,000
lines, with root-relative paths and public root labels only. New routes
list, inspect, follow (SSE) and cancel jobs and start the scan
(POST /prompt_manager/jobs/scan, 409 busy while another record-changing
job runs). The legacy /prompt_manager/scan routes attach to the same job
and stream unchanged payloads.

web/js/scan-progress.js

@@ -1,7 +1,7 @@
 /**
  * State of the output-image scan as seen by the dashboard.
  *
- * The scan runs on the server as a background job (see py/api/scan_job.py) and
+ * The scan runs on the server as a background job (see py/api/jobs.py) and
  * reports progress over SSE. This module turns those events into a small
  * immutable state object and answers the UI questions that depend on it:
  * what dismissing the modal should do, what the minimized pill should say, and