feat(notifications+admin): SMS (Kavenegar) + Email (SMTP) channels & templates

Backend (notification-svc):
- channel_config table (per-tenant Kavenegar + SMTP settings) + migration 18
- sender pkg: Kavenegar SMS client + SMTP mailer (STARTTLS / implicit TLS), stdlib only
- endpoints: GET/PUT /v1/channels[/:channel], POST /v1/sms/send, POST /v1/email/send
  (template + {{var}} rendering); deliveries logged
- seeded 3 Persian email templates: welcome / account_verification / promotion
- gateway routes /v1/{channels,sms,email}/* → notification

Admin UI:
- /admin/messaging: SMS + Email provider config cards, test-send, email template editor
- nav link + fa/en labels

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/notification/internal/models/models.go

@@ -193,3 +193,33 @@ type APIError struct {
 	Code    string `json:"code"`
 	Message string `json:"message"`
 }
+
+// ── Channel provider config (SMS / Email) ────────────────────────────────────
+
+type ChannelConfig struct {
+	TenantID  uuid.UUID      `json:"tenant_id"`
+	Channel   string         `json:"channel"` // 'sms' | 'email'
+	Settings  map[string]any `json:"settings"`
+	Enabled   bool           `json:"enabled"`
+	UpdatedAt time.Time      `json:"updated_at"`
+}
+
+type UpsertChannelConfigRequest struct {
+	Settings map[string]any `json:"settings"`
+	Enabled  *bool          `json:"enabled"`
+}
+
+type SendSMSRequest struct {
+	To      string `json:"to"      binding:"required"`
+	Message string `json:"message" binding:"required"`
+}
+
+type SendEmailRequest struct {
+	To           string            `json:"to" binding:"required"`
+	Subject      string            `json:"subject"`
+	BodyHTML     string            `json:"body_html"`
+	BodyText     string            `json:"body_text"`
+	TemplateCode string            `json:"template_code"`
+	Locale       string            `json:"locale"`
+	Variables    map[string]string `json:"variables"`
+}