- Add ArtifactImportService to parse game JSON and create collection artifacts
- Maps game skill_id to our (group, modifier) format using verified mapping
- Handles skill quality -> strength lookup via ArtifactSkill.base_values
- Supports duplicate detection via game_id, with optional update_existing flag
- Quirk artifacts get proficiency from game data; skills stored as empty
- Add POST /collection/artifacts/import endpoint
- Add game_id column to collection_artifacts, collection_weapons,
collection_summons for tracking game inventory instance IDs
