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>

backend/db/migrations/19_notification_campaigns.sql

@@ -0,0 +1,26 @@
+-- =====================================================================
+-- NOTIFICATION SCHEMA — Part 19: marketing campaigns
+-- A campaign sends an SMS or Email to a user segment (resolved from identity.users)
+-- via the configured channel providers.
+-- =====================================================================
+
+SET search_path TO notification, public;
+
+CREATE TABLE IF NOT EXISTS campaigns (
+    id            UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id     UUID NOT NULL,
+    name          TEXT NOT NULL,
+    channel       TEXT NOT NULL,                 -- 'sms' | 'email'
+    audience      TEXT NOT NULL DEFAULT 'all',   -- 'all' | 'verified' | 'with_plan'
+    subject       TEXT,                          -- email only
+    body_html     TEXT,                          -- email body / sms text
+    template_code TEXT,                          -- optional email template
+    status        TEXT NOT NULL DEFAULT 'Draft', -- Draft | Sending | Sent | Failed
+    total_count   INT  NOT NULL DEFAULT 0,
+    sent_count    INT  NOT NULL DEFAULT 0,
+    failed_count  INT  NOT NULL DEFAULT 0,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    sent_at       TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_campaigns_tenant ON campaigns (tenant_id, created_at DESC);