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/models.py

@@ -90,6 +90,11 @@ class Settings(SQLModel, table=True):
 
     id: Optional[int] = Field(default=1, primary_key=True)
     default_ntfy_server: str = "https://ntfy.sh"
+    # Default-server auth, used as a fallback for feeds without their own and
+    # for the "send test" action (for ntfy servers with access control).
+    default_ntfy_token: str = ""
+    default_ntfy_username: str = ""
+    default_ntfy_password: str = ""
     check_interval: int = 5  # minutes (global default)
 
     # Auth toggle (per-user credentials live in the User table).