Add a daily Docker image-update notification

Dockhand's pending-update counts were only visible if you happened to
open the Docker page. New "Docker image update available" toggle
under Settings -> Notifications, sharing the same daily
time/timezone as the secret and Tailscale key expiry reminders (same
node-schedule reschedule-on-settings-change pattern as those two).
Reads each enabled Dockhand integration's already-cached update-check
results via listContainers() rather than triggering a fresh
per-container registry lookup, so it costs nothing extra beyond what
the Docker page itself already fetches, and lists every container
with an update pending across all environments/integrations in one
notification.

Verified end-to-end against a fake local Dockhand server (one
environment, two containers, one flagged with a pending update) and a
fake webhook receiver on an isolated scratch database: the check
correctly found only the flagged container (with its newerVersion),
sent exactly one notification with the right title/content, excluded
the up-to-date container, and sent nothing at all when the setting
was toggled off despite still finding the same pending update.
Confirmed the real dev database's mtime was untouched throughout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/src/services/notify.ts

@@ -178,6 +178,20 @@ export async function notifySecretExpiry(
   );
 }
 
+export async function notifyDockerUpdates(
+  updatesAvailable: { integrationName: string; containerName: string; environmentName: string; newerVersion: string | null }[],
+): Promise<void> {
+  if (updatesAvailable.length === 0) return;
+  if (!(await eventEnabled("dockerUpdateCheck"))) return;
+  const lines = updatesAvailable.map(
+    (u) => `${u.containerName} [${u.environmentName}, ${u.integrationName}]${u.newerVersion ? ` → ${u.newerVersion}` : ""}`,
+  );
+  await notify(
+    "Homelab Manager — Docker Updates Available",
+    `${updatesAvailable.length} container${updatesAvailable.length !== 1 ? "s have" : " has"} an image update available:\n\n${lines.join("\n")}`,
+  );
+}
+
 export async function notifyTailscaleKeyExpiry(
   expiring: { integrationName: string; deviceLabel: string; daysLeft: number }[],
 ): Promise<void> {