feat(webhook): add workflow_step webhook events for step-level CI notifications

Adds HookEventWorkflowStep event type that fires on every step state
transition (queued -> in_progress -> completed). Follows the same
pattern as the existing workflow_job events.

- New WorkflowStepPayload struct with run/job/step context
- WorkflowStepStatusUpdate notifier interface + dispatch
- Step state change detection in UpdateTask runner endpoint
- Fix: register workflow_step in updateHookEvents API mapping
- Full test coverage mirroring workflow_job tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

options/locale/locale_en-US.json

@@ -2329,6 +2329,8 @@
   "repo.settings.event_workflow_run_desc": "Gitea Actions Workflow run queued, waiting, in progress, or completed.",
   "repo.settings.event_workflow_job": "Workflow Jobs",
   "repo.settings.event_workflow_job_desc": "Gitea Actions Workflow job queued, waiting, in progress, or completed.",
+  "repo.settings.event_workflow_step": "Workflow Steps",
+  "repo.settings.event_workflow_step_desc": "Gitea Actions Workflow step started or completed.",
   "repo.settings.event_package": "Package",
   "repo.settings.event_package_desc": "Package created or deleted in a repository.",
   "repo.settings.branch_filter": "Branch filter",