refactor(backend): introduce DI interfaces for testability

Replace concrete *db.Queries and *pgxpool.Pool dependencies with
db.Querier and db.TxManager interfaces across all handlers, game hub,
and auth. This enables unit testing with mocks.

- Enable sqlc emit_interface to generate Querier interface
- Add TxManager abstraction to encapsulate transactions
- Convert auth package-level functions to Authenticator struct
- Add TaskQueueInterface/TaskWorkerInterface for game.Hub
- Add initial unit tests for game logic and API handlers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 130 insertions, 0 deletions

diff --git a/backend/api/handler_test.go b/backend/api/handler_test.go
new file mode 100644
index 0000000..47ad92c
--- /dev/null
+++ b/backend/api/handler_test.go
@@ -0,0 +1,130 @@
+package api
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"albatross-2026-backend/config"
+	"albatross-2026-backend/db"
+)
+
+// mockQuerier implements db.Querier for testing.
+type mockQuerier struct {
+	db.Querier
+	getGameByIDFunc func(ctx context.Context, gameID int32) (db.GetGameByIDRow, error)
+}
+
+func (m *mockQuerier) GetGameByID(ctx context.Context, gameID int32) (db.GetGameByIDRow, error) {
+	if m.getGameByIDFunc != nil {
+		return m.getGameByIDFunc(ctx, gameID)
+	}
+	return db.GetGameByIDRow{}, pgx.ErrNoRows
+}
+
+// mockTxManager implements db.TxManager for testing.
+type mockTxManager struct{}
+
+func (m *mockTxManager) RunInTx(ctx context.Context, fn func(q db.Querier) error) error {
+	return fn(&mockQuerier{})
+}
+
+// mockGameHub implements GameHubInterface for testing.
+type mockGameHub struct {
+	calcCodeSizeResult int
+	enqueueErr         error
+}
+
+func (m *mockGameHub) CalcCodeSize(_ string, _ string) int {
+	return m.calcCodeSizeResult
+}
+
+func (m *mockGameHub) EnqueueTestTasks(_ context.Context, _, _, _ int, _, _ string) error {
+	return m.enqueueErr
+}
+
+// mockAuthenticator implements AuthenticatorInterface for testing.
+type mockAuthenticator struct {
+	loginResult int
+	loginErr    error
+}
+
+func (m *mockAuthenticator) Login(_ context.Context, _, _ string) (int, error) {
+	return m.loginResult, m.loginErr
+}
+
+func TestPostGamePlaySubmit_GameNotFound(t *testing.T) {
+	h := Handler{
+		q:    &mockQuerier{},
+		txm:  &mockTxManager{},
+		hub:  &mockGameHub{},
+		auth: &mockAuthenticator{},
+		conf: &config.Config{},
+	}
+	user := &db.User{UserID: 1}
+	resp, err := h.PostGamePlaySubmit(context.Background(), PostGamePlaySubmitRequestObject{
+		GameID: 999,
+		Body:   &PostGamePlaySubmitJSONRequestBody{Code: "test"},
+	}, user)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, ok := resp.(PostGamePlaySubmit404JSONResponse); !ok {
+		t.Errorf("expected 404 response, got %T", resp)
+	}
+}
+
+func TestPostGamePlaySubmit_GameNotRunning(t *testing.T) {
+	h := Handler{
+		q: &mockQuerier{
+			getGameByIDFunc: func(_ context.Context, _ int32) (db.GetGameByIDRow, error) {
+				return db.GetGameByIDRow{
+					GameID:   1,
+					Language: "php",
+					StartedAt: pgtype.Timestamp{
+						Valid: false,
+					},
+				}, nil
+			},
+		},
+		txm:  &mockTxManager{},
+		hub:  &mockGameHub{calcCodeSizeResult: 10},
+		auth: &mockAuthenticator{},
+		conf: &config.Config{},
+	}
+	user := &db.User{UserID: 1}
+	resp, err := h.PostGamePlaySubmit(context.Background(), PostGamePlaySubmitRequestObject{
+		GameID: 1,
+		Body:   &PostGamePlaySubmitJSONRequestBody{Code: "<?php echo 1;"},
+	}, user)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if r, ok := resp.(PostGamePlaySubmit403JSONResponse); !ok {
+		t.Errorf("expected 403 response, got %T", resp)
+	} else if r.Message != "Game is not running" {
+		t.Errorf("unexpected message: %s", r.Message)
+	}
+}
+
+func TestPostLogin_AuthFailure(t *testing.T) {
+	h := Handler{
+		q:    &mockQuerier{},
+		txm:  &mockTxManager{},
+		hub:  &mockGameHub{},
+		auth: &mockAuthenticator{loginErr: errors.New("invalid credentials")},
+		conf: &config.Config{},
+	}
+	resp, err := h.PostLogin(context.Background(), PostLoginRequestObject{
+		Body: &PostLoginJSONRequestBody{Username: "user", Password: "wrong"},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, ok := resp.(PostLogin401JSONResponse); !ok {
+		t.Errorf("expected 401 response, got %T", resp)
+	}
+}