Add ability to load go-vcr cassettes to populate mock server with real 3commas API responses. This enables testing against actual production data without manual mocking.

## Features

**Core API:**
- `LoadVCRCassette(path)` - Load single VCR cassette
- `LoadVCRCassettes(...paths)` - Load multiple cassettes

**What Gets Loaded:**
- ✅ Bots with ALL 50+ fields from real API
- ✅ Deals with ALL 100+ fields from real API
- ✅ **bot_events PRESERVED exactly as recorded** (critical for testing)
- ✅ Auto-creates minimal bots when deals reference non-existent bot IDs

**Behavior:**
- Only processes successful (2xx) responses
- Errors on duplicate IDs (ensures clean VCR preparation)
- Skips non-GET requests
- Pattern matches: /ver1/deals/{id}/show, /ver1/deals, /ver1/bots

## Usage Example

```go
func TestWithRealData(t *testing.T) {
    mockServer := server.NewTestServer(t)
    defer mockServer.Close()

    // Load real API response from VCR cassette
    mockServer.LoadVCRCassette("testdata/fixtures/my_deal")

    // Deal now available with ALL real data including bot_events
    resp, _ := http.Get(mockServer.URL() + "/ver1/deals/123/show")
    var deal tcmock.Deal
    json.NewDecoder(resp.Body).Decode(&deal)

    // bot_events preserved from real 3commas API!
    assert.Len(t, deal.BotEvents, 3)
}
```

## Why This Matters

VCR cassettes contain REAL complex data from 3commas that would be tedious to mock manually:
- Complex bot_events messages with actual trading activity
- All 100+ deal fields with real profit/loss calculations
- Actual account names, pairs, and trading strategies

This makes testing more realistic and maintainable.

## Dependencies

- Added gopkg.in/dnaeon/go-vcr.v3 for YAML parsing

## Tests

- TestLoadVCRCassette_SingleDeal - Basic loading and verification
- TestLoadVCRCassette_DuplicateError - Duplicate ID detection
- TestLoadVCRCassettes_Multiple - Loading multiple cassettes
- TestExampleVCRLoading - Example usage pattern