Refactor webpush store tests and add coverage

- Add SetSubscriptionUpdatedAt to Store interface, remove DB() accessor
- Rename store files to store_sqlite.go and store_postgres.go
- Use camelCase for test function names
- Add tests for upsert field updates and multi-user removal
- Use transaction in setupNewPostgresDB
- Use lowercase "excluded." in PostgreSQL upsert query

webpush/store.go

@@ -29,6 +29,7 @@ type Store interface {
 	RemoveSubscriptionsByEndpoint(endpoint string) error
 	RemoveSubscriptionsByUserID(userID string) error
 	RemoveExpiredSubscriptions(expireAfter time.Duration) error
+	SetSubscriptionUpdatedAt(endpoint string, updatedAt int64) error
 	Close() error
 }