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, 0 deletions

diff --git a/justfile b/justfile
index 73f0219..fe4c6b2 100644
--- a/justfile
+++ b/justfile
@@ -56,3 +56,6 @@ check:
     cd backend; just check
     cd worker/swift; just check
     npm -w frontend run check
+
+test:
+    cd backend; just test