Modify periodic loop timing and add batch threshold flush

[Hans37675042]

Summary

• Modify(equipment): rewrite read loop interval using time.monotonic() + ceil() to prevent execution time drift
• Modify(mongo): rewrite read loop interval using time.monotonic() + ceil() to prevent execution time drift
• Fix(mongo): trigger immediate flush via _flush_event when batch size reaches threshold, instead of waiting for next periodic cycle
• Chore: add .obsidian/ to .gitignore

Changed Files

• csp_lib/equipment/device/base.py — rewrite interval calculation
• csp_lib/mongo/uploader.py — flush loop rewrite + add _flush_event mechanism
• tests/mongo/test_uploader.py — add uploader related tests
• .gitignore — add .obsidian/