refactor: move tgdbGameId and igdbGameId to game metadata field

[coderabbitai]

Two game ID fields are currently stored as top-level columns but should be stored in a metadata JSON field instead.

File: src/schemas/game.ts lines 78–79

tgdbGameId: z.number().nullable().optional(), // TODO: store in metadata
igdbGameId: z.number().nullable().optional(), // TODO: For IGDB game creation (stored in metadata for now)

Action required:

• Migrate tgdbGameId and igdbGameId columns to the game's metadata JSON field.
• Update all query/write paths that reference these fields.
• Add a Prisma migration.

---

Requested by @Producdevity — related to PR #375