feat(notifications+admin): marketing campaigns

- campaigns table (migration 19) + CRUD + send endpoint in notification-svc
- audience resolution reads cross-schema from identity.users (all / verified /
  with_plan); send dispatches via the SMS or Email channel and logs deliveries
- endpoints: GET/POST /v1/campaigns, POST /v1/campaigns/:id/send, DELETE
- gateway route /v1/campaigns/* → notification
- /admin/marketing: create campaign (channel, audience, template/subject/body),
  list with status + sent counts, send, delete

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

services/notification/internal/models/models.go

@@ -223,3 +223,31 @@ type SendEmailRequest struct {
 	Locale       string            `json:"locale"`
 	Variables    map[string]string `json:"variables"`
 }
+
+// ── Marketing campaigns ──────────────────────────────────────────────────────
+
+type Campaign struct {
+	ID           uuid.UUID  `json:"id"`
+	TenantID     uuid.UUID  `json:"tenant_id"`
+	Name         string     `json:"name"`
+	Channel      string     `json:"channel"`  // sms | email
+	Audience     string     `json:"audience"` // all | verified | with_plan
+	Subject      *string    `json:"subject,omitempty"`
+	BodyHTML     *string    `json:"body_html,omitempty"`
+	TemplateCode *string    `json:"template_code,omitempty"`
+	Status       string     `json:"status"`
+	TotalCount   int        `json:"total_count"`
+	SentCount    int        `json:"sent_count"`
+	FailedCount  int        `json:"failed_count"`
+	CreatedAt    time.Time  `json:"created_at"`
+	SentAt       *time.Time `json:"sent_at,omitempty"`
+}
+
+type CreateCampaignRequest struct {
+	Name         string  `json:"name"    binding:"required"`
+	Channel      string  `json:"channel" binding:"required"` // sms | email
+	Audience     string  `json:"audience"`
+	Subject      *string `json:"subject"`
+	BodyHTML     *string `json:"body_html"`
+	TemplateCode *string `json:"template_code"`
+}