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/import_.py

@@ -1,12 +1,22 @@
-from fastapi import APIRouter, Depends, Form, Request, UploadFile
+from fastapi import APIRouter, BackgroundTasks, Depends, Form, Request, UploadFile
 from fastapi.responses import RedirectResponse
 from fastapi.templating import Jinja2Templates
 
-from app.db import get_db
+from app.db import SessionLocal, get_db
 from app.models import Playlist
 from app.security.deps import require_auth
 from app.services import manual_import
 
+
+async def _run_import_bg(source: str | None, playlist_name: str | None, imported_by: str) -> None:
+    """Import in the background with its own DB session. The request's session
+    is closed once the response is sent, so we cannot reuse it here."""
+    db = SessionLocal()
+    try:
+        await manual_import.import_track(db, source, playlist_name, imported_by)
+    finally:
+        db.close()
+
 router = APIRouter(prefix="/import", tags=["import"])
 templates = Jinja2Templates(directory="app/templates")
 
@@ -37,13 +47,14 @@ async def upload(
 
 @router.post("/run")
 async def run_import(
+    background: BackgroundTasks,
     source: str = Form(""),
     playlist_name: str = Form(""),
     user: dict = Depends(require_auth),
-    db=Depends(get_db),
 ):
+    # Importing (tag, prep, beets import, rescan) can take a while, so run it in
+    # the background and redirect immediately. Progress and the outcome show up
+    # under Settings then Jobs.
     imported_by = user.get("email") or user.get("sub", "unknown")
-    record = await manual_import.import_track(
-        db, source or None, playlist_name or None, imported_by
-    )
-    return RedirectResponse(url=f"/import?run_id={record.id}", status_code=303)
+    background.add_task(_run_import_bg, source or None, playlist_name or None, imported_by)
+    return RedirectResponse(url="/import?started=1", status_code=303)