Persist deleted task state and clean SSE connections

[shuofengzhang]

What changed

• Persist task deletions by calling save_tasks(tasks) in DELETE /api/task/{task_id}.
• Clear any in-memory SSE connection state for the deleted task to avoid stale queues.

Why

• Deletions were only applied in memory, so removed tasks could reappear after a service restart when tasks.json was reloaded.
• Cleaning task-specific SSE state during deletion prevents lingering in-memory references for removed tasks.

Testing

• scripts/clone_and_test.sh wendy7756/AI-Video-Transcriber
• Tests: not run (no tests found)