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/dedup.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, Request
+from fastapi import APIRouter, BackgroundTasks, Depends, Request
 from fastapi.responses import RedirectResponse
 from fastapi.templating import Jinja2Templates
 
@@ -53,14 +53,15 @@ async def dedup_index(request: Request, user: dict = Depends(require_auth)):
 
 
 @router.post("/scan")
-async def scan(user: dict = Depends(require_auth)):
+async def scan(background: BackgroundTasks, user: dict = Depends(require_auth)):
     # Runs both the file-naming and acoustic passes -- see
-    # dedup_review_service.scan_all(). The candidates table shows which
-    # pass caught each one instead of needing two separate buttons.
-    tag_run, fuzzy_run = await dedup_review_service.scan_all(triggered_by="manual")
-    if tag_run is None or fuzzy_run is None:
-        return RedirectResponse(url="/dedup?skipped=1", status_code=303)
-    return RedirectResponse(url="/dedup", status_code=303)
+    # dedup_review_service.scan_all(). The acoustic pass can take a while on a
+    # large library, so run it in the background and return immediately; the
+    # candidates appear here once it finishes (reload), and the run shows up
+    # under Settings then Jobs. If the pipeline is busy it records skipped_lock
+    # there rather than blocking.
+    background.add_task(dedup_review_service.scan_all, triggered_by="manual")
+    return RedirectResponse(url="/dedup?started=1", status_code=303)
 
 
 @router.post("/confirm")