Background long-running actions; confirm destructive ones

- Run now (jobs and playlists), dedup scan, genre preview/apply, and manual
  import now dispatch via BackgroundTasks and redirect immediately, instead
  of awaiting a job that can run for the better part of an hour and hang the
  browser or reverse proxy. Progress shows in the Jobs runs table (which
  already polls); if the pipeline is busy the run records skipped_lock there.
- Fix the import banner, which claimed work was ongoing after the request
  had actually blocked to completion; it now reflects the backgrounded start.
- Add confirmation prompts to the dedup per-row and bulk delete and to
  "Fix genres now", matching the existing confirms on library and playlist
  deletes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/templates/jobs/index.html

@@ -1,6 +1,9 @@
 {% extends "settings/_layout.html" %}
 {% block settings_title %}Jobs{% endblock %}
 {% block settings_content %}
+{% if request.query_params.get('started') %}
+<div class="notice">Started in the background. Watch the runs table below (it refreshes on its own). If the pipeline is already busy, this run shows as skipped.</div>
+{% endif %}
 <h2 style="margin-top:0;">Maintenance jobs</h2>
 <div class="table-wrap scroll">
   <table>