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/index.ts

@@ -25,12 +25,14 @@ import { searchRouter } from "./routes/search.js";
 import { initSecretExpiryScheduler } from "./services/secretExpiryScheduler.js";
 import { initTailscaleKeyExpiryScheduler } from "./services/tailscaleKeyExpiryScheduler.js";
 import { initLogRetentionScheduler } from "./services/logRetentionScheduler.js";
+import { initDockerUpdateScheduler } from "./services/dockerUpdateScheduler.js";
 
 warnIfAuthNotConfigured();
 await runMigrations();
 await initSecretExpiryScheduler();
 await initTailscaleKeyExpiryScheduler();
 await initLogRetentionScheduler();
+await initDockerUpdateScheduler();
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const webDist = join(__dirname, "..", "..", "web", "dist");