Add default-server ntfy auth (fix 403 on protected topics)

The "send test" action, admin alerts and feeds without their own
credentials now use configurable default-server token / basic auth,
so publishing works against ntfy servers with access control enabled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/schemas.py

@@ -47,6 +47,9 @@ class FeedIn(BaseModel):
 
 class SettingsIn(BaseModel):
     default_ntfy_server: str = "https://ntfy.sh"
+    default_ntfy_token: str = ""
+    default_ntfy_username: str = ""
+    default_ntfy_password: str = ""
     check_interval: int = 5
     auth_enabled: bool = False
     # Telegram