Add multi-provider event discovery and improve sync UI

backend/app/providers/registry.py

@@ -0,0 +1,12 @@
+from app.providers.bandsintown import BandsintownProvider
+from app.providers.eventim import EventimProvider
+from app.providers.ticketmaster import TicketmasterProvider
+
+
+def get_providers():
+    return [
+        TicketmasterProvider(),
+        BandsintownProvider(),
+        EventimProvider(),
+    ]
+