autoresearch iter 16: add batch user lookup and use it for log enrichment

enrichAccessLogsWithUsers preferred the UserService contract over the local
repository — correct layering, but it looped GetUserByID and issued up to a
page-size worth of separate SELECTs against w_users, while the single-query
WHERE id IN variant was only reached in the no-contract fallback branch.
Give the contract a GetUsersByIDs so callers can keep the layering and drop
the N+1. The test asserts 1 query batched against 3 per-id, so the counting
itself is checked.

backend/core/contracts/user.go

@@ -62,6 +62,10 @@ type UserService interface {
 	// GetUserByID retrieves a user by ID.
 	GetUserByID(ctx context.Context, id uint64) (*UserDTO, error)
 
+	// GetUsersByIDs retrieves several users in one round-trip. An empty ids
+	// slice yields no results and touches no storage.
+	GetUsersByIDs(ctx context.Context, ids []uint64) ([]*UserDTO, error)
+
 	// GetUserByUsername retrieves a user by username.
 	GetUserByUsername(ctx context.Context, username string) (*UserDTO, error)