fix(home): "next sync" must reflect the schedule the device is *on*

The card's "next sync" was computed locally as `lastSeenAt + interval`,
which broke the moment the user PATCHed a new interval: the device is
still asleep on whatever schedule was active at its last poll, but the
local record now has the new interval, so we'd display a misleading
"in 2m" after a 5→3 min change.

Fix: server stamps `nextPollExpectedAt` on every poll (200/304/204),
PWA reads it directly. The timestamp doesn't move when settings are
edited — only when the device actually polls and picks up a new
schedule. Same field also drives the settings-sheet "Next update"
preview, which had the same flaw.

Side effects:
- `markSeen()` now flushes on the 204 paths too — they previously
  set lastSeenAt without flushing (latent bug for devices with no
  approved images / missing assets).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/Version20260507230002.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260507230002 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add device.next_poll_expected_at — server stamps it on every poll so the PWA shows the *real* next sync time, unaffected by locally-edited settings that the device has not yet picked up';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE device ADD next_poll_expected_at TIMESTAMP(0) WITHOUT TIME ZONE DEFAULT NULL');
+        $this->addSql("COMMENT ON COLUMN device.next_poll_expected_at IS '(DC2Type:datetime_immutable)'");
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE device DROP COLUMN next_poll_expected_at');
+    }
+}