Implement in-app notification dispatch channel

Model the dual-channel dispatch as a coordinator function (not a class method on FCM dispatcher) that calls the FCM channel and the in-app channel concurrently using Future.wait where possible, then merges the two outcomes into a unified DispatchResult. The key design principle: the two channels are independent — a failure in one must not prevent the other from running. Use try/catch around each channel invocation independently. For the Supabase insert, use upsert with onConflict: 'user_id,event_id' to implement idempotency.

Define a NotificationRecord Dart model with fromJson/toJson and map it to the Supabase row. Keep the in-app channel as a separate injectable class so it can be mocked independently in tests. The coordinator function is what the BLoC event handler calls — it abstracts channel details from business logic.

Unit tests using flutter_test with mocked Supabase client. Required test cases: (1) both channels succeed — result is full_success with both push message_id and in-app record id, (2) FCM succeeds, Supabase fails — result is push_ok_inapp_failed, record not present in mock store, (3) FCM fails, Supabase succeeds — result is push_failed_inapp_ok, in-app record is present, (4) both fail — result is dead_letter, (5) duplicate event_id — second call skips insert and returns existing record id. Integration test against a Supabase test project: verify RLS allows recipient to read inserted record and blocks other users.