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>

src/Controller/DeviceApiController.php

@@ -183,6 +183,7 @@ class DeviceApiController extends AbstractController
             'uniquenessWindow'        => $d->getUniquenessWindow(),
             'linkedAt'                => $d->getLinkedAt()->format(\DateTimeInterface::ATOM),
             'lastSeenAt'              => $d->getLastSeenAt()?->format(\DateTimeInterface::ATOM),
+            'nextPollExpectedAt'      => $d->getNextPollExpectedAt()?->format(\DateTimeInterface::ATOM),
             'lockedImageId'           => $d->getLockedImage()?->getId(),
             'currentImageId'          => $d->getCurrentImage()?->getId(),
         ];

src/Controller/DeviceImageController.php

@@ -61,6 +61,17 @@ class DeviceImageController extends AbstractController
         $currentImageId = (int) $request->headers->get('X-Current-Image-Id', '-1');
         $device->markSeen();
 
+        // Stamp when we expect the device to call back — the PWA reads this
+        // directly so its "next sync" label reflects the schedule the device
+        // is actually on, not the freshly-saved one that won't reach it
+        // until that next poll.
+        $device->setNextPollExpectedAt(
+            (new \DateTimeImmutable())->modify('+' . (int) ceil($intervalMs / 1000) . ' seconds')
+        );
+        // Flush up-front so the 204/no_image/no_asset paths persist these too
+        // (they previously didn't flush at all — latent bug for lastSeenAt).
+        $em->flush();
+
         // Locked image bypasses rotation entirely.
         $image = $device->getLockedImage() ?? $this->rotation->advance($device);
 
@@ -111,9 +122,6 @@ class DeviceImageController extends AbstractController
             // would otherwise have set this. Without the assignment, currentImage
             // stays stale — Home would keep showing the previous photo even
             // though the device has been confirming the new one for cycles.
-            // Also flush so markSeen() above is persisted on every 304 (lastSeenAt
-            // would otherwise freeze whenever the device polls and gets no
-            // change, causing the status badge to drift to "offline").
             $device->setCurrentImage($image);
             $em->flush();
 

src/Entity/Device.php

@@ -71,6 +71,17 @@ class Device
     #[ORM\Column(nullable: true)]
     private ?\DateTimeImmutable $lastSeenAt = null;
 
+    /**
+     * Server-stamped wall-clock time at which the device is expected to poll
+     * next, computed as `now + computeIntervalMs($device)` at every successful
+     * response. The PWA reads this directly so the displayed "next sync"
+     * always reflects the schedule the device is *actually on* — not whatever
+     * the user just saved through the settings sheet (which the device won't
+     * pick up until that next poll).
+     */
+    #[ORM\Column(nullable: true)]
+    private ?\DateTimeImmutable $nextPollExpectedAt = null;
+
     /**
      * Orientation in effect when currentImage was last served as a 200 response.
      * Used alongside currentImage's id to decide whether a poll can be answered
@@ -151,6 +162,9 @@ class Device
     public function getLastSeenAt(): ?\DateTimeImmutable { return $this->lastSeenAt; }
     public function markSeen(): static { $this->lastSeenAt = new \DateTimeImmutable(); return $this; }
 
+    public function getNextPollExpectedAt(): ?\DateTimeImmutable { return $this->nextPollExpectedAt; }
+    public function setNextPollExpectedAt(?\DateTimeImmutable $t): static { $this->nextPollExpectedAt = $t; return $this; }
+
     public function getCurrentImageOrientation(): ?Orientation { return $this->currentImageOrientation; }
     public function setCurrentImageOrientation(?Orientation $o): static { $this->currentImageOrientation = $o; return $this; }