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/routers/jobs.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, HTTPException, Request
+from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException, Request
 from fastapi.responses import PlainTextResponse, RedirectResponse
 from fastapi.templating import Jinja2Templates
 from sqlalchemy import select
@@ -67,15 +67,18 @@ async def runs_table_partial(request: Request, user: dict = Depends(require_auth
 
 
 @router.post("/{job_key:path}/run")
-async def run_now(job_key: str, user: dict = Depends(require_auth)):
+async def run_now(job_key: str, background: BackgroundTasks, user: dict = Depends(require_auth)):
     scheduler = scheduler_service.get_scheduler()
     if scheduler is None:
         raise HTTPException(503, "scheduler not running")
-    try:
-        await scheduler_service.trigger_now(scheduler, job_key)
-    except ValueError as exc:
-        raise HTTPException(404, str(exc))
-    return RedirectResponse(url="/settings/jobs", status_code=303)
+    # Validate the job exists now (so a bad key still 404s), then run it in the
+    # background and redirect immediately. A playlist sync can take the better
+    # part of an hour; awaiting it here would hang the browser/reverse proxy.
+    # Progress shows up in the runs table below, which polls every few seconds.
+    if scheduler.get_job(job_key) is None:
+        raise HTTPException(404, f"no such registered job: {job_key}")
+    background.add_task(scheduler_service.trigger_now, scheduler, job_key)
+    return RedirectResponse(url="/settings/jobs?started=1", status_code=303)
 
 
 @router.post("/{job_key:path}/toggle")