feat: Add Unsplash stock photo integration

[cominop]

Summary

Adds a self-contained Unsplash stock photo integration to BrightBean Studio, allowing users to search and import photos directly from the composer.

What's included

• apps/unsplash/ — new Django app with:
  • Unsplash API client with error handling and required attribution
  • Search endpoint (GET /api/v1/media/unsplash/search/)
  • Import endpoint (POST /api/v1/media/unsplash/import/)
  • Folder management endpoints for organizing imported media
• Frontend modal (Alpine.js) — search Unsplash and import with one click from the composer toolbar
• MediaAsset.external_id field — tracks stock photo provenance (e.g., Unsplash photo ID)
• Workspace.integration_settings JSON field — for per-workspace integration config

Dependencies

• Adds djangorestframework to requirements. The plugin uses DRF for its REST views. Happy to rewrite using Django Ninja if you'd prefer consistency — just let me know.

Configuration

Requires UNSPLASH_ACCESS_KEY in .env (free Developer tier is sufficient). Without it, the integration gracefully degrades — the modal renders empty and API endpoints return clear errors.

Testing

• Manual: composer → "Search Unsplash" button → search → import → media library
• No regressions: no existing files or flows modified beyond the minimal wiring in settings/urls/templates

Screenshots

The integration adds a "Search Unsplash" button next to "Media Library" in the composer toolbar. Clicking it opens a modal with search, pagination, and one-click import with attribution.