Add ntfy authentication support (username/password and API key)

- Settings table gets ntfy_username, ntfy_password, ntfy_api_key columns
- Backend applies Basic or Bearer auth header when sending notifications
- Settings page UI lets you toggle between no auth, basic, or token auth
- Masked credential display on load to avoid exposing stored secrets
- README updated with auth modes documentation

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

mysql/init.sql

@@ -41,7 +41,10 @@ CREATE TABLE IF NOT EXISTS settings (
     ntfy_server VARCHAR(200) DEFAULT 'https://ntfy.sh',
     notification_time VARCHAR(5) DEFAULT '07:00',
     timezone VARCHAR(50) DEFAULT 'UTC',
-    location_name VARCHAR(100)
+    location_name VARCHAR(100),
+    ntfy_username VARCHAR(200),
+    ntfy_password VARCHAR(200),
+    ntfy_api_key  VARCHAR(200)
 );
 
 CREATE TABLE IF NOT EXISTS notification_log (