Show popular fetch phases in nav indicator and Downloads page

- Phase 1 (crawling) now creates the task immediately so Downloads shows it
- Phase label updates to 'Enriching view counts' when phase 2 starts
- Nav bar DownloadIndicator also polls /channels/tasks and shows spinning
  indicator + progress % for background tasks (not just file downloads)

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

backend/routers/channels.py

@@ -680,7 +680,20 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name:
     from ..database import SessionLocal
     from concurrent.futures import ThreadPoolExecutor, as_completed
 
+    task_id = f"popular-{channel_id}"
+    label = f"Popular fetch — {channel_name}" if channel_name else "Popular fetch"
+
     # Phase 1 — flat-playlist: crawl all channel videos quickly
+    with _tasks_lock:
+        _tasks[task_id] = {
+            "id": task_id,
+            "label": label,
+            "phase": "Crawling channel…",
+            "total": 0,
+            "done": 0,
+            "started_at": datetime.utcnow().isoformat(),
+        }
+
     if youtube_channel_id.startswith("@"):
         url = f"https://www.youtube.com/{youtube_channel_id}/videos"
     else:
@@ -750,17 +763,15 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name:
         db.close()
 
     if not video_ids:
+        with _tasks_lock:
+            _tasks.pop(task_id, None)
         return
 
-    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(),
-        }
+        if task_id in _tasks:
+            _tasks[task_id]["phase"] = "Enriching view counts…"
+            _tasks[task_id]["total"] = len(video_ids)
+            _tasks[task_id]["done"] = 0
 
     results = {}
     try: