feat: add MiniMax image backend

[octo-patch]

Reason: Add a configurable MiniMax image backend for the slide rendering pipeline.

Changes:

• Add global and China MiniMax image-generation endpoint selection with supported model and request validation.
• Support aspect-ratio or explicit width and height requests and URL or base64 image responses.
• Route text-to-image rendering through the configured backend while preserving reference-image edits.
• Document environment configuration and add focused request and response tests.

Checks:

• pnpm typecheck
• pnpm test
• pnpm build
• Secret scan