feat: add streak reward and 100-achievement milestone system

backend/CMakeLists.txt

@@ -14,6 +14,7 @@ add_library(csp_core
   src/app_state.cc
   src/services/crypto.cc
   src/services/auth_service.cc
+  src/services/achievement_service.cc
   src/services/experience_service.cc
   src/services/problem_service.cc
   src/services/user_service.cc
@@ -101,6 +102,7 @@ add_executable(csp_tests
   tests/version_test.cc
   tests/sqlite_db_test.cc
   tests/auth_service_test.cc
+  tests/achievement_service_test.cc
   tests/experience_service_test.cc
   tests/auth_http_test.cc
   tests/domain_test.cc