OA-T11: Create Module Struct + Wire into Broker

[devonartis]

Plan ref: Task 11, Milestone 4\n\nCreate internal/oauth/module.go with NewModule(Deps) + RegisterRoutes(mux). Wire into cmd/broker/main.go. Removable: delete import + RegisterRoutes call.\n\nFiles: internal/oauth/module.go, cmd/broker/main.go\nGate: go build -o bin/broker ./cmd/broker && go test ./...