BYOK AI review: dashboard settings UI for AI review + provider key (Option A)

[JSONbored]

Part of #525. The maintainer-facing UI for the BYOK AI review (depends on the backend issue above).

Scope (UI — apps/gittensory-ui)

• In the repo settings dashboard, add an "AI review" section: toggle aiReviewMode (off/advisory/block), byok, and pick provider + model.
• A write-only "provider API key" field that POSTs to the maintainer-authenticated /v1/repos/:owner/:repo/ai-key route (server-side), shows only last4/configured state, and supports rotate/remove. The key is never rendered back.
• Authenticated by the maintainer's GitHub session; only repos they own/maintain.
• Clear copy: free Workers AI default vs BYOK (their key, their provider billed); consensus blocking is always free + confirmed-contributors-only.

Acceptance

A maintainer can enable AI review and configure their own Anthropic/OpenAI key entirely self-serve, without an operator or the internal token.