feat(notices): add system notice infrastructure

Server-side notice registry with per-user condition evaluation (firstLogin,
existingUserBeforeVersion, addonEnabled, dateWindow, role, custom).
Notices are sorted by priority then severity, filtered against dismissals
stored in a new user_notice_dismissals table, and served via
GET /api/system-notices/active + POST /api/system-notices/:id/dismiss.

Client renders notices through a host component that partitions by
display type (modal / banner / toast). The modal renderer supports
multi-page pagination with directional slide transitions, keyboard
navigation, and correct dismiss-all semantics on CTA / X / ESC.
Dismissals are optimistic with a single background retry.

Includes 3.0.0 upgrade notices (v3-photos, v3-journey, v3-features),
onboarding welcome modal, and full i18n coverage across 15 languages.
The /journey route is addon-gated on both client and server.

Also includes: unit + integration test suites, registry integrity test
that validates action CTA IDs against client source, and technical
documentation in docs/system-notices.md.

server/src/routes/systemNotices.ts

@@ -0,0 +1,29 @@
+import { Router } from 'express';
+import { authenticate } from '../middleware/auth.js';
+import { getActiveNoticesFor, dismissNotice } from '../systemNotices/service.js';
+import type { AuthRequest } from '../types.js';
+
+const router = Router();
+
+// GET /api/system-notices/active
+// Returns notices active for the authenticated user.
+router.get('/active', authenticate, (req, res) => {
+  const userId = (req as AuthRequest).user!.id;
+  const notices = getActiveNoticesFor(userId);
+  res.json(notices);
+});
+
+// POST /api/system-notices/:id/dismiss
+// Marks a notice as dismissed for the authenticated user. Idempotent.
+router.post('/:id/dismiss', authenticate, (req, res) => {
+  const userId = (req as AuthRequest).user!.id;
+  const noticeId = req.params.id;
+  const ok = dismissNotice(userId, noticeId);
+  if (!ok) {
+    res.status(404).json({ error: 'NOTICE_NOT_FOUND' });
+    return;
+  }
+  res.status(204).end();
+});
+
+export default router;