feat: add streaming mode support for LLM API

[myc999]

Summary

• Add streaming mode option for LLM API calls
• Some API services only support streaming responses, this option enables compatibility
• New checkbox in settings UI: "启用流式输出 (Stream Mode)"

Changes

• pixelle_video/config/schema.py: Add stream field to LLMConfig
• pixelle_video/config/manager.py: Add stream parameter to get/set methods
• pixelle_video/services/llm_service.py: Add _call_with_stream() method for streaming calls
• web/components/settings.py: Add stream mode toggle checkbox

Test Plan

• Enable stream mode in settings
• Test with streaming-only API services
• Test with standard non-streaming API services (backward compatible)

🤖 Generated with Claude Code

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}