New AI board action create_stock_image. The assistant can now pull real photos onto
the canvas from a prompt — ask "add a photo of a golden retriever" and it searches the free
stock libraries (Unsplash / Pexels / Pixabay / Openverse, best available provider), hosts
the top match, and drops it as an image card. Supports count (1–4) to add several picks at
once, staggered into a small grid so they don't overlap.
Schema + validation in lib/ai/actions.ts (MAX_STOCK_PER_ACTION, query required, count
clamped); documented to the model in lib/ai/board.ts (ACTIONS_SPEC).
Client resolution in Canvas.tsx (resolveStockImages) reuses the exact /api/stock/search
/api/stock/pick endpoints the manual picker uses — search-then-host, fire-and-forget,
failures toast rather than throw. Sign-in gated like all AI + stock features.
