Add dynamic city discovery from DOM.RIA sitemap ...

- Implement src/discovery.py to extract city slugs from sitemap.xml.gz
- Use heuristics to filter out regions, districts, metro stations, streets
- Merge sitemap data with homepage navigation links for completeness
- Update config.py to remove hardcoded CITY_SLUGS, use FALLBACK_CITY_NAMES
- Update main.py to auto-discover cities when --city not provided
- Add --discover-cities flag to list all discovered cities and exit
- Tested with Kiev, Lviv, Odessa multi-city dry-run

Co-Authored-By: Claude <noreply@anthropic.com>