feat(auth): add session-based auth service with tests

backend/tests/auth_service_test.cc

@@ -0,0 +1,23 @@
+#include <catch2/catch_test_macros.hpp>
+
+#include "csp/db/sqlite_db.h"
+#include "csp/services/auth_service.h"
+
+TEST_CASE("auth register/login/verify") {
+  auto db = csp::db::SqliteDb::OpenMemory();
+  csp::db::ApplyMigrations(db);
+
+  csp::services::AuthService auth(db);
+
+  const auto r = auth.Register("alice", "password123");
+  REQUIRE(r.user_id > 0);
+  REQUIRE(r.token.size() > 10);
+
+  const auto uid = auth.VerifyToken(r.token);
+  REQUIRE(uid.has_value());
+  REQUIRE(uid.value() == r.user_id);
+
+  const auto r2 = auth.Login("alice", "password123");
+  REQUIRE(r2.user_id == r.user_id);
+  REQUIRE(r2.token != r.token);
+}