Add Web Push notifications for task assignment

When a task is created or reassigned to a staff member, a push notification
is sent to any devices they have subscribed on. Keys are auto-generated on
first boot and stored in the config table (no manual VAPID setup needed).

- Backend: web-push dep, push_subscriptions table, lib/push.js (VAPID +
  send), routes/push.js (vapid-key / subscribe / unsubscribe endpoints),
  push notify wired into task-core createTask and tasks PATCH reassignment
- Frontend: switched vite-plugin-pwa to injectManifest strategy, custom
  sw.js handles precache + push event + notificationclick, usePushSubscription
  hook requests permission and registers subscription on login

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/db.js

@@ -148,6 +148,18 @@ export async function initDb() {
     );
 
     CREATE INDEX IF NOT EXISTS task_events_task_idx ON task_events (task_id, created_at);
+
+    CREATE TABLE IF NOT EXISTS push_subscriptions (
+      id         SERIAL PRIMARY KEY,
+      user_email TEXT NOT NULL,
+      endpoint   TEXT NOT NULL UNIQUE,
+      p256dh     TEXT NOT NULL,
+      auth       TEXT NOT NULL,
+      created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+      updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+    );
+
+    CREATE INDEX IF NOT EXISTS push_subs_email_idx ON push_subscriptions (user_email);
   `)
 
   await seedDefaults()