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/dedup/index.html

@@ -13,13 +13,14 @@
   </div>
 </div>
 
-{% if request.query_params.get('skipped') %}
-<div class="notice warning">One or both passes didn't run — something else was using the pipeline at that moment (check <a href="/settings/jobs">Jobs</a> for what). Try again in a moment.</div>
+{% if request.query_params.get('started') %}
+<div class="notice">Scan started. It runs in the background; reload this page in a bit to see new candidates, or watch its progress under <a href="/settings/jobs">Jobs</a>.</div>
 {% endif %}
 
 <h2>Pending candidates</h2>
 <p class="muted" style="margin-top:-0.5rem;">Paths are relative to the library root. Hover a name for the full path.</p>
-<form id="bulk-delete-form" method="post" action="/dedup/confirm"></form>
+<form id="bulk-delete-form" method="post" action="/dedup/confirm"
+      onsubmit="return confirm('Permanently delete every selected file from disk? This cannot be undone.')"></form>
 <div class="table-wrap scroll">
   <table class="dedup-table">
     <colgroup>
@@ -39,7 +40,8 @@
         <td class="mono path-cell" title="{{ c.delete_path }}">{{ c.delete_display }}</td>
         <td class="muted">{{ (c.delete_size_bytes / 1024 / 1024) | round(1) if c.delete_size_bytes else '?' }} MB</td>
         <td class="actions-cell">
-          <form method="post" action="/dedup/confirm" class="inline">
+          <form method="post" action="/dedup/confirm" class="inline"
+                onsubmit="return confirm('Permanently delete this file from disk?\n{{ c.delete_display }}')">
             <input type="hidden" name="candidate_id" value="{{ c.id }}">
             <button type="submit" class="btn btn-sm btn-danger">Delete</button>
           </form>