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, 3 insertions, 4 deletions

diff --git a/backend/api/handler_wrapper.go b/backend/api/handler_wrapper.go
index 7448d13..28b89e4 100644
--- a/backend/api/handler_wrapper.go
+++ b/backend/api/handler_wrapper.go
@@ -5,8 +5,6 @@ package api
 import (
 	"context"
 
-	"github.com/jackc/pgx/v5/pgxpool"
-
 	"albatross-2026-backend/config"
 	"albatross-2026-backend/db"
 )
@@ -17,12 +15,13 @@ type HandlerWrapper struct {
 	impl Handler
 }
 
-func NewHandler(queries *db.Queries, pool *pgxpool.Pool, hub GameHubInterface, conf *config.Config) *HandlerWrapper {
+func NewHandler(queries db.Querier, txm db.TxManager, hub GameHubInterface, auth AuthenticatorInterface, conf *config.Config) *HandlerWrapper {
 	return &HandlerWrapper{
 		impl: Handler{
 			q:    queries,
-			pool: pool,
+			txm:  txm,
 			hub:  hub,
+			auth: auth,
 			conf: conf,
 		},
 	}