Add local settings management

Add cellular-first ntfy delivery with WiFi fallback and safe SIM868 HTTP header framing.

Co-authored-by: Copilot <[email protected]>

modules/app_sql.py

@@ -405,6 +405,24 @@ def set_setting(key, value):
         conn.commit()
 
 
+def set_settings(settings):
+    """Persist a validated group of settings atomically."""
+    conn = get_conn()
+    if conn is None:
+        raise RuntimeError("Settings database is not available.")
+    with _conn_lock:
+        try:
+            conn.executemany(
+                "INSERT INTO settings (key, value) VALUES (?, ?) "
+                "ON CONFLICT(key) DO UPDATE SET value = excluded.value",
+                settings.items(),
+            )
+            conn.commit()
+        except sqlite3.Error:
+            conn.rollback()
+            raise
+
+
 def get_alert_states():
     conn = get_conn()
     if conn is None: