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/db/migrations.ts

@@ -1605,6 +1605,20 @@ function runMigrations(db: Database.Database): void {
         CREATE INDEX IF NOT EXISTS idx_idempotency_keys_created ON idempotency_keys(created_at);
       `);
     },
+    // Migration 101: System notices — user tracking columns + dismissals table
+    () => {
+      db.exec(`ALTER TABLE users ADD COLUMN first_seen_version TEXT NOT NULL DEFAULT '0.0.0'`);
+      db.exec(`ALTER TABLE users ADD COLUMN login_count INTEGER NOT NULL DEFAULT 0`);
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS user_notice_dismissals (
+          user_id      INTEGER NOT NULL,
+          notice_id    TEXT    NOT NULL,
+          dismissed_at INTEGER NOT NULL,
+          PRIMARY KEY (user_id, notice_id),
+          FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+        )
+      `);
+    },
   ];
 
   if (currentVersion < migrations.length) {