feat(steam): import Steam library metadata into Notion

[MaximilianMauroner]

Steam games require manual entry even though the account library and store metadata are available through Steam APIs.

Proposed change

Import owned games for a configured Steam account and enrich them with store metadata before writing them to Notion.

Acceptance criteria

• The importer reads the account library with a Steam API key.
• Imported games include app ID, name, artwork, playtime, and available store metadata.
• Private profiles and missing store records produce clear errors.
• Repeated runs update existing games without duplicates.