feat: add MiniMax image-to-image generation

[octo-patch]

Reason: Add MiniMax image-to-image support to the existing artwork generation scripts.

• Add a typed image generation client with regional endpoints, request validation, and URL or base64 response handling.
• Use subject references in the announcement banner and Discord icon generators while preserving their existing fallback.
• Add focused tests for validation, regional routing, response formats, and API failures.

Checks:

• Secret scan completed without findings.

[vercel]

@octo-patch is attempting to deploy a commit to the Crafter Station Team on Vercel.

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