Stock photo search — a new bottom-toolbar button opens a picker that searches
Unsplash, Pexels, Pixabay, and Openverse. Pick an image and it's downloaded and
hosted in your space's Storage bucket, then dropped on the board with a photographer
credit line (satisfying each provider's attribution terms; Unsplash's download ping is
fired on use). Provider API keys live server-side (UNSPLASH_ACCESS_KEY,
PEXELS_API_KEY, PIXABAY_API_KEY); Openverse needs no key, so search works out of
the box, and the keyed providers appear as their keys are configured. Sign-in gated
(hosting needs Storage). New lib/stock.ts, app/api/stock/search, app/api/stock/pick,
components/StockImagePicker.tsx; ImageCard gained an optional credit.
