feat: Excel template export with meeting number auto-generation

- Add meeting_number field (M-YYYYMMDD-XX format) with auto-generation
- Refactor Excel export to use cell coordinates instead of placeholders
- Export files saved to backend/record/ directory with meeting number filename
- Add database migration for meeting_number column
- Add start.sh script for managing frontend/backend/sidecar services
- Update OpenSpec documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/schemas.py

@@ -96,6 +96,7 @@ class MeetingUpdate(BaseModel):
 class MeetingResponse(BaseModel):
     meeting_id: int
     uuid: str
+    meeting_number: Optional[str] = None
     subject: str
     meeting_time: datetime
     location: Optional[str] = None