Async Download to Library with progress; Download to client

Backend:
- DownloadTask model + background thread runner: streams the file with
  progress (%, bytes, speed) and a cancel flag, then indexes it, names it
  J-<id>.<ext> and applies the e621 metadata
- DownloadTaskViewSet (create/retrieve/cancel) replaces the synchronous
  endpoint; the status footer's worker counts now reflect download jobs
- Client download proxy (/api/online/file/) streams an e621 original to
  the browser with Content-Disposition: attachment, restricted to the
  configured e621 CDN hosts so it cannot be used as an open proxy

Frontend:
- Online detail: progress bar with percentage, transferred size, speed
  and cancel while downloading; success links to the new J-ID
- New 'Download to client' button available to everyone (guests too)

backend/apps/library/downloads.py

@@ -0,0 +1,122 @@
+"""Background 'Download to Library' jobs with progress tracking."""
+
+import logging
+import threading
+import time
+from pathlib import Path
+from urllib.parse import urlparse
+
+from django.conf import settings
+from django.utils import timezone
+
+from . import services
+from .models import DownloadTask
+
+logger = logging.getLogger(__name__)
+
+
+def start_download_task(task_id):
+    thread = threading.Thread(target=run_download_task, args=(task_id,), daemon=True)
+    thread.start()
+
+
+def run_download_task(task_id):
+    task = DownloadTask.objects.filter(id=task_id).first()
+    if task is None:
+        return
+
+    folder = Path(settings.WATCHED_FOLDER)
+    name = (
+        task.filename
+        or Path(urlparse(task.url).path).name
+        or f"post-{task.post_id or 'download'}"
+    )
+    destination = services.unique_destination(folder, name)
+
+    state = {
+        "progress_at": 0.0,
+        "bytes_at": 0,
+        "cancel_at": 0.0,
+        "cancelled": False,
+    }
+
+    def should_cancel():
+        now = time.monotonic()
+        if now - state["cancel_at"] >= 1.0:
+            state["cancelled"] = DownloadTask.objects.filter(
+                id=task_id, cancelled=True
+            ).exists()
+            state["cancel_at"] = now
+        return state["cancelled"]
+
+    def on_progress(downloaded, total):
+        now = time.monotonic()
+        if now - state["progress_at"] < 0.5 and (total == 0 or downloaded < total):
+            return
+        elapsed = max(now - state["progress_at"], 0.001)
+        speed = None
+        if state["bytes_at"] > 0 and downloaded >= state["bytes_at"]:
+            speed = (downloaded - state["bytes_at"]) / elapsed
+        state["progress_at"] = now
+        state["bytes_at"] = downloaded
+        DownloadTask.objects.filter(id=task_id).update(
+            downloaded=downloaded,
+            total=total,
+            progress=int(downloaded * 100 / total) if total else 0,
+            speed=speed,
+            updated_at=timezone.now(),
+        )
+
+    DownloadTask.objects.filter(id=task_id).update(
+        status=DownloadTask.STATUS_DOWNLOADING, updated_at=timezone.now()
+    )
+    try:
+        services.download_file(
+            task.url,
+            destination,
+            progress_callback=on_progress,
+            should_cancel=should_cancel,
+        )
+        item, _, location, _ = services.index_file(destination, folder)
+        services.rename_location_to_j_id(item, location)
+
+        update_fields = []
+        if item.uploaded_by_id is None and task.user_id is not None:
+            item.uploaded_by = task.user
+            update_fields.append("uploaded_by")
+        if task.e621_data:
+            item.e621_post_id = task.post_id
+            item.e621_data = task.e621_data
+            update_fields += ["e621_post_id", "e621_data"]
+            rating = (task.e621_data or {}).get("rating")
+            if not item.rating and rating in {"s", "q", "e"}:
+                item.rating = rating
+                update_fields.append("rating")
+        if update_fields:
+            item.save(update_fields=update_fields + ["updated_at"])
+
+        DownloadTask.objects.filter(id=task_id).update(
+            status=DownloadTask.STATUS_COMPLETE,
+            progress=100,
+            speed=None,
+            library_item=item,
+            updated_at=timezone.now(),
+        )
+    except services.DownloadCancelled:
+        destination.unlink(missing_ok=True)
+        DownloadTask.objects.filter(id=task_id).update(
+            status=DownloadTask.STATUS_CANCELLED,
+            progress=0,
+            downloaded=0,
+            speed=None,
+            updated_at=timezone.now(),
+        )
+    except Exception as exc:  # noqa: BLE001 - report background failures
+        destination.unlink(missing_ok=True)
+        logger.exception("Download task %s failed", task_id)
+        DownloadTask.objects.filter(id=task_id).update(
+            status=DownloadTask.STATUS_ERROR,
+            error=str(exc),
+            speed=None,
+            updated_at=timezone.now(),
+        )