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>

openspec/changes/archive/2025-12-11-update-excel-template-export/specs/excel-export/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Template-based Generation
+The Excel export SHALL use openpyxl with template files and cell coordinate mapping.
+
+#### Scenario: Cell coordinate filling
+- **WHEN** Excel is generated from template `meeting_template.xlsx`
+- **THEN** data SHALL be filled into specific cells:
+  - D3: meeting subject
+  - D4: meeting time
+  - D5: chairperson
+  - F4: location
+  - F5: recorder
+  - D6: attendees (comma separated)
+  - C8: meeting number (M-YYYYMMDD-XX format)
+  - D8: conclusions (newline separated if multiple)
+
+#### Scenario: Dynamic row filling for action items
+- **WHEN** meeting has action items
+- **THEN** rows SHALL be filled starting from row 10:
+  - Column C: action item system code (A-YYYYMMDD-XX)
+  - Column D: action item content
+  - Column F: owner
+  - Column G: due date
+  - Column H: status (Open/In Progress/Done/Delayed)
+
+#### Scenario: Template path resolution
+- **WHEN** export is requested
+- **THEN** server SHALL load template from `backend/template/meeting_template.xlsx`
+
+### Requirement: Complete Data Inclusion
+The exported Excel SHALL include all meeting metadata, conclusions, and action items.
+
+#### Scenario: Full metadata export
+- **WHEN** Excel is generated
+- **THEN** it SHALL include subject, meeting_time, location, chairperson, recorder, attendees, and meeting_number
+
+#### Scenario: Conclusions export
+- **WHEN** Excel is generated with multiple conclusions
+- **THEN** all conclusions SHALL be merged with newline separator into cell D8
+
+#### Scenario: Action items export with status
+- **WHEN** Excel is generated
+- **THEN** all action items SHALL be listed with system_code, content, owner, due_date, and status
+
+## ADDED Requirements
+
+### Requirement: Meeting Number Generation
+The system SHALL automatically generate meeting numbers for each meeting record.
+
+#### Scenario: Auto-generate meeting number
+- **WHEN** a new meeting is created
+- **THEN** system SHALL generate meeting number in format `M-YYYYMMDD-XX` where XX is daily sequence
+
+#### Scenario: Meeting number uniqueness
+- **WHEN** multiple meetings are created on same date
+- **THEN** each SHALL receive unique sequential number (M-20251211-01, M-20251211-02, etc.)
+
+#### Scenario: Meeting number in export
+- **WHEN** Excel is exported
+- **THEN** meeting_number SHALL be displayed in cell C8