Wire up visitor metrics from webserver DB and fix uptime monitoring

- Read visitors from /server_storage/visitors.db (webserver's DB) instead of
  admin dash's own table; geoip lookups at query time for globe markers
- Globe card now shows 24h, 7d, and all-time unique visitor counts
- Uptime monitor: Nextcloud via host.docker.internal for Docker networking,
  Website and Gitea monitored on public domains
- UptimeCard uses real hourly history bars instead of Math.random() mock
- docker-compose: mount /server_storage:ro, add extra_hosts for Linux compat

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/visitors-db.ts

@@ -0,0 +1,25 @@
+import sqlite3 from 'sqlite3';
+import { open, Database } from 'sqlite';
+
+let db: Database | null = null;
+
+/**
+ * Opens a read-only connection to the webserver's visitors.db.
+ * Returns null if the DB doesn't exist yet (webserver hasn't created it).
+ * Retries on each call until the DB becomes available.
+ */
+export async function getVisitorsDb(): Promise<Database | null> {
+  if (db) return db;
+
+  try {
+    db = await open({
+      filename: '/server_storage/visitors.db',
+      driver: sqlite3.Database,
+      mode: sqlite3.OPEN_READONLY,
+    });
+    return db;
+  } catch {
+    // DB doesn't exist yet — don't cache the failure so we retry next time
+    return null;
+  }
+}