feat: implement IGDB age ratings check for adult content detection

[coderabbitai]

The current IGDB adult content detection only checks themes (e.g., IGDB_ADULT_THEME_IDS). Age rating data (PEGI 18, ESRB M/AO) is not yet used.

File: src/server/igdb.ts lines 185–186

Action required:

• Fetch age_ratings data from the IGDB API when checking for adult content.
• Extend the check to flag content with PEGI 18 or ESRB M/AO ratings.

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Requested by @Producdevity — related to PR #375