fix: avoid blocking audio writes

[Sanjays2402]

Description

Moves TTS response writes off the asyncio event loop so concurrent presentation requests are not stalled by synchronous file I/O.

Motivation and Context

Fixes #1603.

Screenshots

N/A

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Focused async regression test, Ruff check/format, and Python compilation pass.

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR fixes a performance issue where synchronous file I/O operations for TTS audio responses were blocking the asyncio event loop, preventing concurrent presentation requests from being processed. The fix introduces a new utility function write_bytes that uses asyncio.to_thread to offload file writes to a thread pool, and replaces the blocking open().write() call in the TTS chunk generation with this non-blocking alternative.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/app/utils/file_io.py
2	surfsense_backend/tests/unit/agents/video_presentation/test_file_io.py
3	surfsense_backend/app/agents/video_presentation/nodes.py

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