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>

templates/watch.html

@@ -66,6 +66,7 @@
             <a href="{{ video.video_url }}" target="_blank" class="btn btn-link">Watch on YouTube</a>
             {% if video.download_status.value == 'completed' and video.download_path %}
                 <a href="/api/video/stream/{{ video.id }}?download=1" class="btn btn-secondary" download>Download MP4</a>
+                <button class="btn btn-danger" onclick="deleteFromDisk()">Delete from Disk</button>
             {% endif %}
         </div>
     </div>
@@ -102,6 +103,26 @@
         startDownload();
     }
 
+    function deleteFromDisk() {
+        if (!confirm('Are you sure you want to delete this video from disk? This cannot be undone.')) return;
+
+        fetch('/api/videos/{{ video.id }}/file', {
+            method: 'DELETE'
+        })
+        .then(response => response.json())
+        .then(data => {
+            if (data.status === 'success') {
+                alert('Video deleted from disk! This page will refresh automatically.');
+                setTimeout(() => location.reload(), 2000);
+            } else {
+                alert('Failed to delete video: ' + data.message);
+            }
+        })
+        .catch(error => {
+            alert('Error: ' + error);
+        });
+    }
+
     // Auto-refresh if video is downloading
     {% if video.download_status.value == 'downloading' %}
         setTimeout(() => location.reload(), 10000); // Refresh every 10 seconds