feat(social): implement Pinterest pin posting

[thejaytang]

Summary

• replace the Pinterest social adapter stub with a real Pinterest API v5 Create Pin call
• map sh1pt social posts to image_url Pin payloads with board sections, alt text, destination links, and hashtag descriptions
• support video Pins when the caller provides a pre-uploaded Pinterest media id and cover image URL
• add social contract coverage plus focused mocked API tests for success, validation, video payloads, and Pinterest errors

Context

Closes #253
References #6

Pinterest API docs used:

• https://developer.pinterest.com/docs/work-with-organic-content-and-users/create-boards-and-pins/

Validation

• corepack pnpm --filter @profullstack/sh1pt-social-pinterest typecheck
• corepack pnpm vitest run packages/social/pinterest/src/index.test.ts
• corepack pnpm --filter @profullstack/sh1pt-social-pinterest build