feat(phase-4): SMS gateway dashboard + stats + health + retry

Backend (backend/app/routers/sms.py):
- GET /api/sms/stats: aggregate totals by status, daily volume chart
  (configurable N days), per-school top-10 breakdown, delivery rate,
  live queue depth
- POST /api/sms/jobs/{id}/retry: reset failed/cancelled job to pending
- GET /api/sms/health: Semaphore API connectivity check + credit balance
- POST /api/sms/trigger-queue: manually fire sms.process_queue Celery task

Frontend (frontend/src/pages/SmsPage.vue) — rebuilt from scratch:
- Semaphore health badge with live status indicator and balance display
- 'Process Queue' button calling POST /api/sms/trigger-queue
- KPI row: Total, Sent, Failed, Pending (queue depth), Delivery Rate %;
  period picker switching between 7/30/90 day views
- Volume bar chart: daily sent vs failed bars with hover tooltips,
  x-axis date labels, legend; no charting library required
- School breakdown panel: top 10 schools by volume with delivery
  percentage bars
- Jobs table: status + school + phone number filters; inline error
  message on failed rows; Retry button for failed/cancelled jobs;
  retry count display; pagination

api.ts: getSmsStats, retrySmsJob, getSmsHealth, triggerSmsQueue

PAUL: Phase 4 marked complete, STATE + ROADMAP updated

.paul/ROADMAP.md

@@ -19,8 +19,8 @@ SMS flow: On-prem TapTrack polls Hub every 30s → Hub queues SMS jobs → Hub s
 ## Current Milestone
 
 **v1.0 — Foundation & Core Services**
-Status: Phase 3 complete — Phase 4 next
-Phases: 3 of 15 complete
+Status: Phase 4 complete — Phase 5 next
+Phases: 4 of 15 complete
 
 ---
 
@@ -31,7 +31,7 @@ Phases: 3 of 15 complete
 | 1     | Project Setup & Infrastructure | 1   | ✅ Complete  | 2026-03-15 |
 | 2     | School Registry + License Mgmt | 2   | ✅ Complete  | 2026-03-15 |
 | 3     | On-Prem License Validation     | 1   | ✅ Complete  | 2026-03-15 |
-| 4     | SMS Gateway (credits + queue)  | TBD | Not started  | —          |
+| 4     | SMS Gateway (credits + queue)  | 2   | ✅ Complete  | 2026-03-15 |
 | 5     | On-Prem SMS Polling Agent      | TBD | Not started  | —          |
 | 6     | Super Admin Dashboard UI       | TBD | Not started  | —          |
 | 7     | School Admin Portal UI         | TBD | Not started  | —          |