image_service.py: Fetches thumbnail from Wikipedia REST API (free,
no account). Falls back to emoji for abstract words (40+ mapped).
Auto-enrichment: When a learning unit is created, images are
automatically fetched for all words that don't have one yet.
Manual endpoint: POST /api/vocabulary/enrich-images fills images
for existing words without images.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
