feat: Add Telnyx SMS integration

Telnyx selected for SMS delivery:
- 50% cheaper than Twilio ($0.004/SMS vs $0.0079)
- Owned infrastructure (better reliability)
- International number support
- Free expert support

Client handles OTP delivery with proper error handling.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

internal/sms/telnyx/client.go

···
+package telnyx
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+const (
+	TelnyxAPIBaseURL = "https://api.telnyx.com/v2"
+	SendMessagePath  = "/messages"
+)
+
+// Client handles communication with Telnyx API
+type Client struct {
+	apiKey             string
+	messagingProfileID string
+	fromNumber         string
+	httpClient         *http.Client
+}
+
+// NewClient creates a new Telnyx client
+func NewClient(apiKey, messagingProfileID, fromNumber string) *Client {
+	return &Client{
+		apiKey:             apiKey,
+		messagingProfileID: messagingProfileID,
+		fromNumber:         fromNumber,
+		httpClient: &http.Client{
+			Timeout: 10 * time.Second,
+		},
+	}
+}
+
+// SendOTP sends an OTP code via SMS
+func (c *Client) SendOTP(ctx context.Context, phoneNumber, code string) error {
+	message := fmt.Sprintf("Your Coves verification code is: %s\n\nThis code expires in 10 minutes.", code)
+
+	req := &SendMessageRequest{
+		From:               c.fromNumber,
+		To:                 phoneNumber,
+		Text:               message,
+		MessagingProfileID: c.messagingProfileID,
+	}
+
+	reqBody, err := json.Marshal(req)
+	if err != nil {
+		return fmt.Errorf("failed to marshal request: %w", err)
+	}
+
+	httpReq, err := http.NewRequestWithContext(ctx, "POST", TelnyxAPIBaseURL+SendMessagePath, bytes.NewReader(reqBody))
+	if err != nil {
+		return fmt.Errorf("failed to create request: %w", err)
+	}
+
+	httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
+	httpReq.Header.Set("Content-Type", "application/json")
+
+	resp, err := c.httpClient.Do(httpReq)
+	if err != nil {
+		return fmt.Errorf("failed to send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+
+	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusCreated {
+		var telnyxErr TelnyxErrorResponse
+		if err := json.Unmarshal(body, &telnyxErr); err == nil && len(telnyxErr.Errors) > 0 {
+			return fmt.Errorf("telnyx API error: %s (code: %s)", telnyxErr.Errors[0].Detail, telnyxErr.Errors[0].Code)
+		}
+		return fmt.Errorf("telnyx API error: status %d, body: %s", resp.StatusCode, string(body))
+	}
+
+	var msgResp SendMessageResponse
+	if err := json.Unmarshal(body, &msgResp); err != nil {
+		return fmt.Errorf("failed to parse response: %w", err)
+	}
+
+	// Check if message was accepted
+	if msgResp.Data.ID == "" {
+		return fmt.Errorf("message not accepted by Telnyx")
+	}
+
+	return nil
+}
+
+// SendMessageRequest represents a Telnyx send message request
+type SendMessageRequest struct {
+	From               string `json:"from"`
+	To                 string `json:"to"`
+	Text               string `json:"text"`
+	MessagingProfileID string `json:"messaging_profile_id"`
+}
+
+// SendMessageResponse represents a Telnyx send message response
+type SendMessageResponse struct {
+	Data MessageData `json:"data"`
+}
+
+// MessageData represents message data in Telnyx response
+type MessageData struct {
+	ID     string `json:"id"`
+	Status string `json:"status"`
+}
+
+// TelnyxErrorResponse represents an error response from Telnyx
+type TelnyxErrorResponse struct {
+	Errors []TelnyxError `json:"errors"`
+}
+
+// TelnyxError represents a single error from Telnyx
+type TelnyxError struct {
+	Code   string `json:"code"`
+	Detail string `json:"detail"`
+	Title  string `json:"title"`
+}