Fix per-resource settings: tags saving, is_public bug, global setting inheritance

Bug fixes:
- tags field was missing from INSERT/UPDATE SQL — tags never persisted
- is_public: `0 !== false` is true in JS, so private resources (is_public=0)
  were silently flipped to public on every edit; fixed with explicit 0/false check

Global setting inheritance:
- One-time DB migration nulls out check_interval and timeout on all existing
  resources so they inherit from global monitoring settings going forward
- Scheduler reads global check_interval and timeout from DB each cycle and
  uses them as the fallback when per-resource values are null
- Create and edit modals: empty field = use global; placeholder shows the
  current global value so it's clear what will be used

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

server/database.js

@@ -333,4 +333,14 @@ db.exec(`
   CREATE INDEX IF NOT EXISTS idx_resources_heartbeat_token ON resources(heartbeat_token);
 `);
 
+// One-time migration: null out per-resource check_interval and timeout so all resources
+// inherit from the global monitoring settings. Runs once, tracked by a settings flag.
+try {
+  const alreadyMigrated = db.prepare("SELECT value FROM settings WHERE key = 'migrated_global_defaults_v1'").get();
+  if (!alreadyMigrated) {
+    db.prepare("UPDATE resources SET check_interval = NULL, timeout = NULL").run();
+    db.prepare("INSERT OR IGNORE INTO settings (key, value) VALUES ('migrated_global_defaults_v1', 'true')").run();
+  }
+} catch (e) {}
+
 module.exports = db;