feat: Export functionality for CSV and JSON formats

[rlaope]

Problem

Users cannot export captured event data for:

• External analysis in tools like Excel, pandas, or Grafana
• Sharing with team members
• Archiving for later review
• Integration with other monitoring systems

Proposed Solution

Add export functionality supporting multiple formats:

1. CSV export for spreadsheet analysis
2. JSON export for programmatic processing
3. Filtered exports (by time range, event type)
4. Download from dashboard UI

Acceptance Criteria

• Add /export API endpoint
• Support CSV format with configurable columns
• Support JSON format (array of events)
• Support JSON Lines format for streaming
• Add time range filtering (from, to parameters)
• Add event type filtering
• Add "Export" button to dashboard
• Include metadata header in exports

API Design

GET /export?format=csv&from=2024-01-01T00:00:00Z&to=2024-01-02T00:00:00Z&type=PINNED

Response Headers:
Content-Type: text/csv
Content-Disposition: attachment; filename="argus-export-2024-01-01.csv"

CSV Format

timestamp,event_type,thread_id,thread_name,carrier_thread,duration_ns
2024-01-01T00:00:00.123Z,VIRTUAL_THREAD_START,123,VT-1,1,
2024-01-01T00:00:00.456Z,VIRTUAL_THREAD_END,123,VT-1,1,333000000
2024-01-01T00:00:00.789Z,VIRTUAL_THREAD_PINNED,124,VT-2,2,

JSON Format

{
  "exportedAt": "2024-01-01T12:00:00Z",
  "query": {
    "from": "2024-01-01T00:00:00Z",
    "to": "2024-01-02T00:00:00Z",
    "type": null
  },
  "totalEvents": 15234,
  "events": [
    {
      "timestamp": "2024-01-01T00:00:00.123Z",
      "eventType": "VIRTUAL_THREAD_START",
      "threadId": 123,
      "threadName": "VT-1",
      "carrierThread": 1
    }
  ]
}

UI Mockup

┌─────────────────────────────────────────────────────────────┐
│ Export Data                                         [X]     │
├─────────────────────────────────────────────────────────────┤
│ Format:    ○ CSV  ○ JSON  ○ JSON Lines                     │
│                                                             │
│ Time Range:                                                 │
│   From: [2024-01-01 00:00] To: [2024-01-02 00:00]          │
│                                                             │
│ Event Types:                                                │
│   [x] START  [x] END  [x] PINNED  [ ] SUBMIT_FAILED        │
│                                                             │
│                              [Cancel]  [Download Export]    │
└─────────────────────────────────────────────────────────────┘

Technical Considerations

• Stream large exports to avoid memory issues
• Consider compression for large exports (gzip)
• Rate limit export requests
• Add export to recent events buffer if persistence is disabled