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, 40 insertions, 0 deletions

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)
+}