Add popular fetch progress to Downloads page

- Track active background tasks in an in-memory dict with a lock
- Expose GET /api/channels/tasks returning running task list
- _fetch_popular_task updates done count as each video fetch completes
- Downloads page polls /tasks every 2s and shows progress bars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/channels.py

@@ -1,4 +1,5 @@
 import json
+import threading as _threading
 from datetime import datetime, timedelta
 from typing import Optional
 
@@ -14,6 +15,9 @@ from ..services import ytdlp
 
 router = APIRouter()
 
+_tasks: dict = {}
+_tasks_lock = _threading.Lock()
+
 
 class ChannelOut(BaseModel):
     id: int
@@ -286,6 +290,12 @@ def sync_all_channels(
     return {"indexing": len(channels)}
 
 
+@router.get("/tasks")
+def get_active_tasks(current_user: User = Depends(get_current_user)):
+    with _tasks_lock:
+        return list(_tasks.values())
+
+
 @router.post("/mark-seen", status_code=204)
 def mark_channels_seen(
     db: Session = Depends(get_db),
@@ -652,11 +662,11 @@ def fetch_popular_videos(
 ):
     """Fetch the channel's most popular videos from YouTube and index them."""
     channel = _get_channel_or_404(db, channel_id)
-    background_tasks.add_task(_fetch_popular_task, channel_id, channel.youtube_channel_id)
+    background_tasks.add_task(_fetch_popular_task, channel_id, channel.youtube_channel_id, channel.name or "")
     return {"detail": "Fetching popular videos"}
 
 
-def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
+def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name: str = ""):
     """Half-and-half popular fetch.
 
     Phase 1 (fast): flat-playlist crawl of the full channel → store any
@@ -742,15 +752,32 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
     if not video_ids:
         return
 
-    with ThreadPoolExecutor(max_workers=3) as pool:
-        futures = {pool.submit(ytdlp.fetch_video_metadata, vid): vid for vid in video_ids}
-        results = {}
-        for future in as_completed(futures):
-            vid = futures[future]
-            try:
-                results[vid] = future.result()
-            except Exception:
-                pass
+    task_id = f"popular-{channel_id}"
+    with _tasks_lock:
+        _tasks[task_id] = {
+            "id": task_id,
+            "label": f"Popular fetch — {channel_name}" if channel_name else "Popular fetch",
+            "total": len(video_ids),
+            "done": 0,
+            "started_at": datetime.utcnow().isoformat(),
+        }
+
+    results = {}
+    try:
+        with ThreadPoolExecutor(max_workers=3) as pool:
+            futures = {pool.submit(ytdlp.fetch_video_metadata, vid): vid for vid in video_ids}
+            for future in as_completed(futures):
+                vid = futures[future]
+                try:
+                    results[vid] = future.result()
+                except Exception:
+                    pass
+                with _tasks_lock:
+                    if task_id in _tasks:
+                        _tasks[task_id]["done"] += 1
+    finally:
+        with _tasks_lock:
+            _tasks.pop(task_id, None)
 
     db = SessionLocal()
     try: