Feature Request: Add incremental transcript processing for the Local backend so AI notes can be refreshed every 5 min #123

Add incremental transcript processing for the Local backend so AI notes can be refreshed every 5 minutes during a recording.

User-facing goal:

1. The app should periodically checkpoint the transcript every 5 minutes while recording.
2. Each checkpoint should trigger AI notes generation from the new transcript since the last checkpoint.
3. Notes generation should be incremental, reusing the existing AI notes and only appending/updating with new transcript content from the latest checkpoint.

Done looks like:

• Local recordings produce updated AI notes without waiting for the recording to stop.
• The notes remain consistent across multiple checkpoints instead of being regenerated from scratch each time.
• Users can see the notes evolve during a long meeting with minimal delay or duplication.

Business Context

We already have a Local backend that writes transcript.md and note.md on disk after recording, and the codebase has existing local notes retry/finalize flows plus design docs for on-device transcription and notes generation. This request extends that flow to support long recordings better and reduce the latency gap between transcript capture and useful notes for the Local/offline experience.

Implementation Guidelines

• Only build the feature or fix the bug described above
• Do not add root-level documentation markdown unless explicitly requested in the feature description
• Do not add testing plan documentation unless explicitly requested in the feature description
• Focus on delivering the user-facing functionality described above