Add a Backer Concierge assistant for catalog questions

[github-actions]

Problem statement

Filtering and sorting help backers who already know what they're looking for. Plenty of people arrive without that clarity — they ask things like "which game suits people who like git puns?" or "what's the highest rated strategy game?" in Discord and email, and someone answers by hand.

Proposed solution

A Backer Concierge assistant should answer those questions on the site, grounded in the Tailspin catalog so it never invents games or numbers. The catalog records title, description, category, publisher and star rating — and nothing else, so the assistant needs to be honest about what it cannot answer (funding totals, backer counts, player counts, prices, release dates) and ask a clarifying question instead of guessing.

Acceptance criteria

• Backers can ask free-text questions about the catalog and get game recommendations
• Every recommendation is grounded in the Tailspin catalog — no invented games, publishers, categories, or star ratings
• The assistant refuses to state funding totals, backer counts, pledge tiers, prices, or release dates, and says why
• Vague requests, or requests that depend on data the catalog doesn't track, get one short clarifying question rather than a guess
• General board-game background may come from a web search, clearly marked as general context rather than a Tailspin fact
• The assistant is reachable from the site with a link in the main navigation
• The interface follows the project's accessibility guidelines (keyboard navigation, ARIA, visible focus states, role="status" for the loading state) and includes data-testid attributes
• Playwright e2e tests cover asking a question and rendering the answer, with the assistant call stubbed
• The catalog export used for grounding is generated by a script and covered by Vitest unit tests