feat: add bounded n8n video workflow

[acedatacloud-dev]

Why

Provide an async video-generation template with a bounded polling contract instead of an unbounded loop.

What

• adds built-in n8n nodes for submit, wait, poll, terminal success, terminal failure, and timeout
• supports prompt and optional image input
• stores no credential ID, token, pinned response, or user data
• adds contract tests and a pinned n8n 2.38.2 import/export CI gate on Node 24 with QuickJS

Evidence

• 9 focused tests pass
• JSON parse and Python compile pass
• isolated n8n 2.38.2 import/export produced exactly one workflow with ID acedata-video-generation-poll-v1
• isolated SQLite credential count was zero

Rollout / rollback

Users import the JSON manually and attach their own Header Auth credential. Revert this PR to withdraw the template.

Out of scope

No workflow nodes or billable API calls were executed. No external marketplace submission or publication is performed here.

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