Add Web Push notifications for backgrounded/frozen browser tabs

The SSE + in-page toast/sound system only works while the tab's JS is
actually running — Chrome (and other browsers) freeze a backgrounded
tab's JS after a stretch of inactivity to save power, so notifications
silently stop regardless of how correct the SSE/toast code is. A Service
Worker is the only mechanism that keeps receiving events independent of
the tab's own lifecycle, so this adds a real Web Push pipeline:

- New `push_subscriptions` table (one row per browser/device a user has
  subscribed from; endpoint is unique so re-subscribing overwrites
  rather than accumulating stale rows).
- VAPID keypair config (lib/push/vapid.ts) — reads
  VAPID_PUBLIC_KEY/VAPID_PRIVATE_KEY/VAPID_SUBJECT from the environment;
  push is silently disabled (never throws) if they're unset, matching
  this codebase's existing "an optional integration outage must never
  break the core flow" pattern (see the LDAP auth comment).
- public/sw.js: a minimal service worker — `push` -> showNotification,
  `notificationclick` -> focus or open the ticket.
- API routes: GET /api/push/vapid-public-key (client needs it to call
  pushManager.subscribe), POST/DELETE /api/push/subscribe.
- lib/push/client.ts: registers the service worker and subscribes,
  called from the notification bell after granting permission and again
  on mount for returning users who already granted it.
- lib/tickets/service.ts: appendMessage() now also fires a push (fire-
  and-forget, never awaited by the caller) to every admin plus the
  ticket's assignee whenever a *customer* message arrives — same "who
  should know" rule as the in-page toast (lib/tickets/visibility.ts).
  Failed sends are inspected: a 404/410 (push service no longer
  recognizes the subscription) prunes the row; anything else is just
  logged, since it might be transient.

Verified server-side end-to-end on this VM: saved a subscription via the
API, created a customer ticket, and confirmed the push attempt actually
fires (web-push validated and rejected a deliberately-malformed test
key, proving the send path is wired correctly) without blocking or
crashing ticket creation. Couldn't verify the full real-browser
subscribe-and-receive path or an actual OS popup from here — this VM has
no desktop/notification service, and headless Chromium's Notification
permission can't be reliably granted in this sandbox (unrelated to the
app code); that last mile needs verifying on a real machine.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012o9j9RezxbZVKQMrB7oRLY

src/lib/db/migrations/meta/_journal.json

@@ -43,6 +43,13 @@
       "when": 1785138258070,
       "tag": "0005_colorful_gorilla_man",
       "breakpoints": true
+    },
+    {
+      "idx": 6,
+      "version": "6",
+      "when": 1786130418783,
+      "tag": "0006_big_daredevil",
+      "breakpoints": true
     }
   ]
 }