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>

2 files changed, 103 insertions, 0 deletions

diff --git a/backend/db/querier.go b/backend/db/querier.go
new file mode 100644
index 0000000..89d4b55
--- /dev/null
+++ b/backend/db/querier.go
@@ -0,0 +1,63 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.30.0
+
+package db
+
+import (
+	"context"
+)
+
+type Querier interface {
+	AddMainPlayer(ctx context.Context, arg AddMainPlayerParams) error
+	AggregateTestcaseResults(ctx context.Context, submissionID int32) (string, error)
+	CreateGame(ctx context.Context, arg CreateGameParams) (int32, error)
+	CreateProblem(ctx context.Context, arg CreateProblemParams) (int32, error)
+	CreateSession(ctx context.Context, arg CreateSessionParams) error
+	CreateSubmission(ctx context.Context, arg CreateSubmissionParams) (int32, error)
+	CreateTestcase(ctx context.Context, arg CreateTestcaseParams) (int32, error)
+	CreateTestcaseResult(ctx context.Context, arg CreateTestcaseResultParams) error
+	CreateUser(ctx context.Context, username string) (int32, error)
+	CreateUserAuth(ctx context.Context, arg CreateUserAuthParams) error
+	DeleteExpiredSessions(ctx context.Context) error
+	DeleteSession(ctx context.Context, sessionID string) error
+	DeleteTestcase(ctx context.Context, testcaseID int32) error
+	GetGameByID(ctx context.Context, gameID int32) (GetGameByIDRow, error)
+	GetLatestState(ctx context.Context, arg GetLatestStateParams) (GetLatestStateRow, error)
+	GetLatestStatesOfMainPlayers(ctx context.Context, gameID int32) ([]GetLatestStatesOfMainPlayersRow, error)
+	GetProblemByID(ctx context.Context, problemID int32) (Problem, error)
+	GetQualifyingRanking(ctx context.Context, arg GetQualifyingRankingParams) ([]GetQualifyingRankingRow, error)
+	GetRanking(ctx context.Context, gameID int32) ([]GetRankingRow, error)
+	GetSubmissionByID(ctx context.Context, submissionID int32) (Submission, error)
+	GetSubmissionsByGameID(ctx context.Context, gameID int32) ([]Submission, error)
+	GetTestcaseByID(ctx context.Context, testcaseID int32) (Testcase, error)
+	GetTestcaseResultsBySubmissionID(ctx context.Context, submissionID int32) ([]TestcaseResult, error)
+	GetUserAuthByUsername(ctx context.Context, username string) (GetUserAuthByUsernameRow, error)
+	GetUserByID(ctx context.Context, userID int32) (User, error)
+	GetUserBySession(ctx context.Context, sessionID string) (User, error)
+	GetUserIDByUsername(ctx context.Context, username string) (int32, error)
+	ListAllGames(ctx context.Context) ([]Game, error)
+	ListGameStateIDs(ctx context.Context) ([]ListGameStateIDsRow, error)
+	ListMainPlayers(ctx context.Context, dollar_1 []int32) ([]ListMainPlayersRow, error)
+	ListProblems(ctx context.Context) ([]Problem, error)
+	ListPublicGames(ctx context.Context) ([]ListPublicGamesRow, error)
+	ListSubmissionIDs(ctx context.Context) ([]int32, error)
+	ListTestcases(ctx context.Context) ([]Testcase, error)
+	ListTestcasesByGameID(ctx context.Context, gameID int32) ([]Testcase, error)
+	ListTestcasesByProblemID(ctx context.Context, problemID int32) ([]Testcase, error)
+	ListUsers(ctx context.Context) ([]User, error)
+	RemoveAllMainPlayers(ctx context.Context, gameID int32) error
+	SyncGameStateBestScoreSubmission(ctx context.Context, arg SyncGameStateBestScoreSubmissionParams) error
+	UpdateCode(ctx context.Context, arg UpdateCodeParams) error
+	UpdateCodeAndStatus(ctx context.Context, arg UpdateCodeAndStatusParams) error
+	UpdateGame(ctx context.Context, arg UpdateGameParams) error
+	UpdateGameStartedAt(ctx context.Context, arg UpdateGameStartedAtParams) error
+	UpdateGameStateStatus(ctx context.Context, arg UpdateGameStateStatusParams) error
+	UpdateProblem(ctx context.Context, arg UpdateProblemParams) error
+	UpdateSubmissionStatus(ctx context.Context, arg UpdateSubmissionStatusParams) error
+	UpdateTestcase(ctx context.Context, arg UpdateTestcaseParams) error
+	UpdateUser(ctx context.Context, arg UpdateUserParams) error
+	UpdateUserIconPath(ctx context.Context, arg UpdateUserIconPathParams) error
+}
+
+var _ Querier = (*Queries)(nil)
diff --git a/backend/db/txmanager.go b/backend/db/txmanager.go
new file mode 100644
index 0000000..9495288
--- /dev/null
+++ b/backend/db/txmanager.go
@@ -0,0 +1,40 @@
+package db
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+type TxManager interface {
+	RunInTx(ctx context.Context, fn func(q Querier) error) error
+}
+
+type PgxTxManager struct {
+	pool    *pgxpool.Pool
+	queries *Queries
+}
+
+func NewTxManager(pool *pgxpool.Pool, queries *Queries) *PgxTxManager {
+	return &PgxTxManager{pool: pool, queries: queries}
+}
+
+func (m *PgxTxManager) RunInTx(ctx context.Context, fn func(q Querier) error) error {
+	tx, err := m.pool.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := tx.Rollback(ctx); err != nil && err != pgx.ErrTxClosed {
+			slog.Error("failed to rollback transaction", "error", err)
+		}
+	}()
+
+	qtx := m.queries.WithTx(tx)
+	if err := fn(qtx); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
+}