Add comprehensive deletion functionality and scheduled cleanup

Features:
- Delete entire channels with all videos and downloaded files
- Delete individual video files while keeping database entries
- Scheduled automatic cleanup of videos older than 7 days
- Proper cascading deletes with file cleanup

Channel Deletion:
- New DELETE endpoint at /api/channels/<id>
- Removes channel, all video entries, and downloaded files
- User ownership verification
- Returns count of deleted files
- UI button on channels page with detailed confirmation dialog

Video File Deletion:
- New DELETE endpoint at /api/videos/<id>/file
- Celery async task to remove file from disk
- Resets download status to pending (allows re-download)
- UI button on watch page for completed videos
- Confirmation dialog with clear warnings

Scheduled Cleanup:
- Celery beat configuration for periodic tasks
- cleanup_old_videos task runs daily at midnight
- Automatically deletes videos completed more than 7 days ago
- Removes files and resets database status
- scheduled_tasks.py for beat schedule configuration
- verify_schedule.py helper to check task scheduling

UI Improvements:
- Added .btn-danger CSS class (black/white theme)
- Delete buttons with loading states
- Detailed confirmation dialogs warning about permanent deletion
- Dashboard now filters to show only completed videos

Bug Fixes:
- Fixed navbar alignment issues
- Added proper error handling for file deletion

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

download_service.py

@@ -163,3 +163,112 @@ def download_videos_batch(video_ids: list[int]) -> dict:
         "total_queued": len(results),
         "tasks": results
     }
+
+
+@celery_app.task(base=DatabaseTask, bind=True)
+def delete_video_file(self, video_id: int) -> dict:
+    """Delete a downloaded video file and reset its download status.
+
+    Args:
+        video_id: Database ID of the VideoEntry
+
+    Returns:
+        Dictionary with deletion result information
+    """
+    session = self.session
+
+    # Get video entry from database
+    video = session.query(VideoEntry).filter_by(id=video_id).first()
+    if not video:
+        return {"error": f"Video ID {video_id} not found"}
+
+    # Check if video has a download path
+    if not video.download_path:
+        return {"error": "Video has no download path", "video_id": video_id}
+
+    # Delete the file if it exists
+    deleted = False
+    if os.path.exists(video.download_path):
+        try:
+            os.remove(video.download_path)
+            deleted = True
+        except OSError as e:
+            return {
+                "error": f"Failed to delete file: {str(e)}",
+                "video_id": video_id,
+                "path": video.download_path
+            }
+
+    # Reset download status and metadata
+    video.download_status = DownloadStatus.PENDING
+    video.download_path = None
+    video.download_completed_at = None
+    video.file_size = None
+    video.download_error = None
+    session.commit()
+
+    return {
+        "video_id": video_id,
+        "status": "deleted" if deleted else "reset",
+        "message": "File deleted and status reset" if deleted else "Status reset (file not found)"
+    }
+
+
+@celery_app.task(base=DatabaseTask, bind=True)
+def cleanup_old_videos(self) -> dict:
+    """Clean up videos older than 7 days.
+
+    Returns:
+        Dictionary with cleanup results
+    """
+    from datetime import timedelta
+
+    session = self.session
+
+    # Calculate cutoff date (7 days ago)
+    cutoff_date = datetime.utcnow() - timedelta(days=7)
+
+    # Query videos that are completed and older than 7 days
+    old_videos = session.query(VideoEntry).filter(
+        VideoEntry.download_status == DownloadStatus.COMPLETED,
+        VideoEntry.download_completed_at < cutoff_date
+    ).all()
+
+    deleted_count = 0
+    failed_count = 0
+    results = []
+
+    for video in old_videos:
+        if video.download_path and os.path.exists(video.download_path):
+            try:
+                os.remove(video.download_path)
+                # Reset download status
+                video.download_status = DownloadStatus.PENDING
+                video.download_path = None
+                video.download_completed_at = None
+                video.file_size = None
+                video.download_error = None
+                deleted_count += 1
+                results.append({
+                    "video_id": video.id,
+                    "title": video.title,
+                    "status": "deleted"
+                })
+            except OSError as e:
+                failed_count += 1
+                results.append({
+                    "video_id": video.id,
+                    "title": video.title,
+                    "status": "failed",
+                    "error": str(e)
+                })
+
+    session.commit()
+
+    return {
+        "total_processed": len(old_videos),
+        "deleted_count": deleted_count,
+        "failed_count": failed_count,
+        "cutoff_date": cutoff_date.isoformat(),
+        "results": results
+    }