Add a privacy policy and update data handling docs

[PAMulligan]

Description

Publish a privacy policy and document how data flows now that Optia has a backend. Cover what the extension sends to the Optia backend and to OpenAI, what is stored (licenses, usage counters), what is not stored, and how billing data is handled entirely by Stripe.

Why

Once requests leave the browser for our backend and OpenAI, we take on data-handling responsibilities and the Chrome Web Store requires a privacy policy and accurate data-safety disclosures. Clear docs build user trust, satisfy review, and keep us honest about never touching card data (Stripe handles it).

Acceptance Criteria

• Privacy policy page published and linked from the extension and store listing
• Documents data sent to the backend (entitlement, coarse usage) and to OpenAI (prompt context)
• States what is stored and retention, and that page content is not retained beyond the request
• States that billing is handled by Stripe and no card data touches Optia
• Explains BYO-key mode (key stays local, goes straight to OpenAI)
• Contact/data-request path included
• Repo docs updated to match (README and docs links)
• CI passes

Resources

• app/src/lib/docs-links.ts and app/src/components/Footer.tsx (where docs/links are referenced)
• Depends on: AI proxy (Build the hosted AI proxy with usage metering and rate limits #7), Stripe billing (Integrate Stripe billing: Checkout, customer portal, and webhooks #5)

[PAMulligan]

Provider decision affecting this policy (2026-07-19): all AI is Anthropic (Claude), not OpenAI — the policy should name Anthropic as the AI processor for hosted requests. BYO-key mode is direct-to-provider and now enforced server-side (the backend never reads a caller key — PMDevSolutions/optia-backend#17), so the policy's claim "key stays local, goes straight to the provider" is accurate as written once "OpenAI" becomes "Anthropic".