# Task 03: Cast API Client

## Objective
Build `internal/cast/client.go` — a typed HTTP client for the Cast.ph SMS API.

## Reference
Read `CAST_API_REFERENCE.md` for exact request/response shapes.

## Types (`internal/cast/types.go`)

```go
type SendRequest struct {
    To       string `json:"to"`
    Message  string `json:"message"`
    SenderID string `json:"sender_id,omitempty"`
}

type SendResponse struct {
    Success   bool   `json:"success"`
    MessageID string `json:"message_id,omitempty"`
    Parts     int    `json:"parts,omitempty"`
    Error     string `json:"error,omitempty"`
}
```

## Client (`internal/cast/client.go`)

### Client struct
```go
type Client struct {
    baseURL    string
    apiKey     string
    senderID   string // default sender ID, can be empty
    httpClient *http.Client
}

func NewClient(baseURL, apiKey, senderID string) *Client
```

### Methods

```go
func (c *Client) SendSMS(ctx context.Context, to, message string) (*SendResponse, error)
```

- POST to `/api/sms/send`
- Set headers: `X-API-Key`, `Content-Type: application/json`
- Body: `{ "to": to, "message": message, "sender_id": c.senderID }` (omit sender_id if empty)
- On non-200: return `CastAPIError` with status code and error message from body
- On 200 with `success: false`: return `CastAPIError` with the error field
- On 200 with `success: true`: return the response
- Retry on 429: max 3 retries, read `Retry-After` header, backoff 1s/2s/4s

### Error type

```go
type CastAPIError struct {
    StatusCode int
    APIError   string
}

func (e *CastAPIError) Error() string {
    return fmt.Sprintf("cast api error (HTTP %d): %s", e.StatusCode, e.APIError)
}
```

### Key behaviors
- HTTP client timeout: 30 seconds
- `sender_id` omitted from JSON when Client.senderID is empty
- Response body always read and closed, even on errors
- Retry on 429 only (not on 5xx — Cast should handle that)
- Log retries with `slog.Warn`

## Acceptance Criteria
- [ ] `go build ./cmd/server/` succeeds
- [ ] `SendSMS` calls correct URL with correct headers
- [ ] `X-API-Key` header set on every request
- [ ] `sender_id` omitted from JSON when empty
- [ ] `CastAPIError` returned on non-200 with statusCode + apiError
- [ ] `CastAPIError` returned on 200 with `success: false`
- [ ] Retry on 429 with backoff (max 3)
- [ ] Context passed to HTTP request
- [ ] HTTP client has 30s timeout