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/migrations/001_add_meeting_number.sql

@@ -0,0 +1,9 @@
+-- Migration: Add meeting_number column to meeting_records table
+-- Date: 2025-12-11
+-- Description: Add meeting_number field for M-YYYYMMDD-XX format
+
+ALTER TABLE meeting_records
+ADD COLUMN meeting_number VARCHAR(20) NULL AFTER uuid;
+
+-- Create index for faster lookups
+CREATE INDEX idx_meeting_number ON meeting_records(meeting_number);