feat: add streak reward and 100-achievement milestone system

backend/tests/sqlite_db_test.cc

@@ -55,4 +55,5 @@ TEST_CASE("migrations create core tables") {
   REQUIRE(CountTable(db.raw(), "source_crystal_settings") == 1);
   REQUIRE(CountTable(db.raw(), "source_crystal_accounts") == 1);
   REQUIRE(CountTable(db.raw(), "source_crystal_transactions") == 1);
+  REQUIRE(CountTable(db.raw(), "user_achievement_milestones") == 1);
 }