docs: add OpenRouter Zero Data Retention (ZDR) enforcement to docs

[devin-ai-integration]

Summary

Documents that ZDR (Zero Data Retention) has been enabled on our OpenRouter account. Updates 4 files across docs and legal:

• docs/pro/2.cloud.mdx — Added bold callout about ZDR enforcement in the OpenRouter Privacy Policy section; updated the data retention row in the policy table
• docs/faq/10.ai-models-and-privacy.mdx — Added ZDR mention to the Pro curated models bullet under LLM data flow
• docs/faq/3.technical.mdx — Appended ZDR sentence to the "How does Char route cloud LLM requests?" answer
• legal/dpa.mdx — Added ZDR to OpenRouter's safeguards column in the sub-processor table

Review & Testing Checklist for Human

• DPA wording: The DPA is a legal document. Verify the added claim ("Zero Data Retention (ZDR) enforced on our account") is appropriate for a legal context and accurately reflects the current OpenRouter account configuration.
• Accuracy of "cannot store even temporarily": The cloud services doc says providers "cannot store your prompts or completions, even temporarily." Confirm this matches OpenRouter's ZDR semantics (their docs say ZDR providers "will not store your data for any period of time" — note: OpenRouter considers in-memory prompt caching acceptable under ZDR).
• Render check: Preview the 4 changed MDX files to confirm markdown tables and bold/link formatting render correctly on the site.

Notes

• Requested by @yujonglee
• Devin session

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