refactor(code-executor): migrate user scripts to single context argument

- replace (page, context, helpers) signature with async (context) => {}
- add ScriptContext interface exposing page, browser, env, getEnv,
  getCredential, getStepOutput, runSnippet, dumpDom, log/warn/error
- rename getEnvUrl to getEnv throughout service and docs
- fix step ordering: normalizeStepOrder and run step rows are now 1-based
- migrate existing DB rows (scenario_steps, scenario_run_steps) +1
- update all tests and stored snippet/step code in DB to new API

server/src/scenario/scenario.service.ts

@@ -104,8 +104,8 @@ export class ScenarioService {
     });
     for (let i = 0; i < ordered.length; i += 1) {
       const step = ordered[i];
-      if (step.order !== i) {
-        step.order = i;
+      if (step.order !== i + 1) {
+        step.order = i + 1;
         await this.stepRepo.save(step);
       }
     }
@@ -120,7 +120,7 @@ export class ScenarioService {
     const created = await this.stepRepo.save(
       this.stepRepo.create({
         ...dto,
-        order: currentCount,
+        order: currentCount + 1,
         scenarioId,
         title: dto.title ?? null,
         execCode: dto.execCode ?? null,