feat(backend): add sqlite migrations + app state + tests

backend/src/app_state.cc

@@ -0,0 +1,22 @@
+#include "csp/app_state.h"
+
+#include <stdexcept>
+
+namespace csp {
+
+AppState& AppState::Instance() {
+  static AppState inst;
+  return inst;
+}
+
+void AppState::Init(const std::string& sqlite_path) {
+  db_ = std::make_unique<db::SqliteDb>(db::SqliteDb::OpenFile(sqlite_path));
+  csp::db::ApplyMigrations(*db_);
+}
+
+csp::db::SqliteDb& AppState::db() {
+  if (!db_) throw std::runtime_error("AppState not initialized");
+  return *db_;
+}
+
+}  // namespace csp